UA-.NETStandard/docs/MigrationGuide.md

# Migration Guide

Use this guide to migrate an application between versions of the OPC UA
.NET Standard Stack. Detailed, version-specific instructions live in the
[`migrate/`](migrate/) subfolder. This page links to those guides and
covers cross-cutting changes.

## Contents

- [General principles](#general-principles)
- [Per-version migration index](#per-version-migration-index)
- [Migrating code that used the exposed diagnostics locks](#migrating-code-that-used-the-exposed-diagnostics-locks)
  - [Why there is no `[Obsolete]` shim](#why-there-is-no-obsolete-shim)
- [Migrating code that used ILocalNode.DataLock](#migrating-code-that-used-ilocalnodedatalock)
- [Migrating code that used BaseVariableValue.Lock](#migrating-code-that-used-basevariablevaluelock)
- [Migrating code that locked on a NodeState or a NodeBrowser](#migrating-code-that-locked-on-a-nodestate-or-a-nodebrowser)
- [Migrating code that used ApplicationConfiguration.PropertiesLock](#migrating-code-that-used-applicationconfigurationpropertieslock)
- [Migrating node types that override FindChild or CreateChild](#migrating-node-types-that-override-findchild-or-createchild)
- [Adopting replica-consistent NodeIds](#adopting-replica-consistent-nodeids)
- [Removed members on ISession](#removed-members-on-isession)
- [Awaiting custom node-manager cleanup](#awaiting-custom-node-manager-cleanup)
- [Migrating code that called IServerInternal.Set* mutators](#migrating-code-that-called-iserverinternalset-mutators)
- [Migrating IServerStartupTask implementations to IServerContext](#migrating-iserverstartuptask-implementations-to-iservercontext)
- [Removed members on IServerInternal](#removed-members-on-iserverinternal)
- [Migrating servers that relied on unserved history advertisement](#migrating-servers-that-relied-on-unserved-history-advertisement)
- [Migrating custom ISessionManager implementations to ShutdownAsync](#migrating-custom-isessionmanager-implementations-to-shutdownasync)
- [Configuring distributed address-space storage](#configuring-distributed-address-space-storage)
- [Migrating SamplingGroupManager create/modify overrides](#migrating-samplinggroupmanager-createmodify-overrides)
- [Migrating callers of the synchronous MonitoredNode2 notification wrappers](#migrating-callers-of-the-synchronous-monitorednode2-notification-wrappers)
- [Migrating callers of the SecurityPolicies lookup and cryptography statics](#migrating-callers-of-the-securitypolicies-lookup-and-cryptography-statics)
- [Migrating code that drove the server subscription publish pipeline](#migrating-code-that-drove-the-server-subscription-publish-pipeline)
- [Migrating channel subclasses that guarded state with DataLock](#migrating-channel-subclasses-that-guarded-state-with-datalock)
- [Transport resource limits](#transport-resource-limits)
- [Migrating channel subclasses that override HandleIncomingMessage](#migrating-channel-subclasses-that-override-handleincomingmessage)
- [Migrating custom IUserDatabase implementations](#migrating-custom-iuserdatabase-implementations)
- [Reading certificates from SecureChannelContext](#reading-certificates-from-securechannelcontext)
- [Write service Value semantics follow OPC 10000-3 and OPC 10000-4](#write-service-value-semantics-follow-opc-10000-3-and-opc-10000-4)
- [Address space permissions, Call, NodeManagement and locales follow OPC 10000-3](#address-space-permissions-call-nodemanagement-and-locales-follow-opc-10000-3)
- [Migrating from 1.05.377 to 1.05.378](#migrating-from-105377-to-105378)
  - [Asynchronous as default](#asynchronous-as-default)
  - [Observability](#observability)
- [Migrating from 1.04 to 1.05](#migrating-from-104-to-105)
- [Support](#support)

## General principles

1. All API that is replaced with newer API is marked `[Obsolete]` and
   code should compile and work albeit of the warnings (which can be
   suppressed). `[Obsolete]` API will be cleaned up in the next *minor*
   version increment. We therefore recommend upgrading from minor
   version to minor version and fixing all `[Obsolete]` warnings as you
   go along.
2. API that cannot be supported anymore will be removed in a minor
   version and migration steps documented in the version sub-folder.
   We try to keep this to an absolute minimum.
3. Bugs or issues found in obsoleted API are not supported.
4. We follow semver, but do not use the major version indicator to
   denote breaking changes like (1) or (2) as we should if we followed
   related conventions. We are a small team and cannot afford to
   maintain previous major versions, therefore we try to keep cases of
   (2) to a minimum and expect you to upgrade to the next minor version
   within 6 months of release.

> **Pro TIP.** Point your favourite coding agent at this guide and let
> it do the migration work for you. The
> [`opcua-v20-migration`](../.agents/skills/opcua-v20-migration/SKILL.md)
> agent skill knows when to load which sub-doc and runs the
> migration-analyzer codefixer end-to-end.

Copilot CLI discovers the skill automatically inside a clone. To install it
for use in any repository:

```bash
copilot plugin marketplace add OPCFoundation/UA-.NETStandard
copilot plugin install opcua-v20-migration@opcua-dotnet
```

## Per-version migration index

| From | To | Where to read |
| --- | --- | --- |
| `1.5.378` | `2.0.x` | [`migrate/2.0.x/`](migrate/2.0.x/README.md) — landing page and symptom-based thematic sub-doc index. |
| `1.05.377` | `1.05.378` | [§ inline below](#migrating-from-105377-to-105378) — small enough to keep on this page. |
| `1.04` | `1.05` | [§ inline below](#migrating-from-104-to-105) — small enough to keep on this page. |

Looking for the broader narrative (non-prescriptive overview of what
changed in a release)? See
[What's New in 2.0](WhatsNewIn2.0.md).

## Migrating code that used the exposed diagnostics locks

`IServerInternal`, `ISession` and `ISubscription` no longer expose their
synchronization primitives. The removed members are:

| Interface | Removed |
| --- | --- |
| `IServerInternal` | `DiagnosticsLock`, `DiagnosticsWriteLock` |
| `ISession` | `DiagnosticsLock` |
| `ISubscription` | `DiagnosticsLock`, `DiagnosticsWriteLock` |

A caller could not reason about these locks: it could not see what else took
them, in what order, or for how long, and holding one across a call back into
the stack could deadlock. Each owner now applies the mutation itself, so the
critical section stays inside the object that understands it.

```csharp
// was
lock (server.DiagnosticsLock)
{
    server.ServerDiagnostics.RejectedSessionCount++;
}

// now
server.UpdateServerDiagnostics(diagnostics => diagnostics.RejectedSessionCount++);
```

The same shape applies to sessions and subscriptions:

```csharp
session.UpdateDiagnostics(diagnostics => diagnostics.ClientLastContactTime = now);
subscription.UpdateDiagnostics(diagnostics => diagnostics.NextSequenceNumber = next);
```

To read a value derived from the session or subscription diagnostics, use the
read counterpart, which holds the same lock for the duration of the projection:

```csharp
uint count = session.ReadDiagnostics(diagnostics => diagnostics.RepublishRequestCount);
```

**Do not let the diagnostics object escape the callback.** Once the callback
returns the lock is released, so any field read from a captured reference is
unsynchronized. Project the values you need inside the callback and return
those.

`IServerInternal.ServerDiagnostics` was removed for the same reason: it handed
out the mutable structure that the lock protects. `UpdateServerDiagnostics` and
the diagnostic node manager are the supported paths.

Analyzer `UA0024` flags each removed member and names its replacement. It
reports rather than auto-fixes: turning a `lock` statement body into a lambda is
not a transformation that can be applied safely without understanding what the
body captures and returns.

### Why there is no `[Obsolete]` shim

Every other removal in this guide keeps an `[Obsolete]` member for a release.
These do not, and deliberately so. A lock is only useful if it is *the* lock the
owner takes. A shim would have to hand back either a lock nobody else takes -
silently turning a working critical section into no synchronization at all - or
the real lock, which is exactly the coupling being removed. A missing member is
a compile error the analyzer explains; a shim would be a race that shows up in
production. `ISession` and `ISubscription` are also implemented by downstream
code, and re-adding an interface member would break every implementer.

## Migrating code that used ILocalNode.DataLock

`ILocalNode.DataLock` (implemented by `Node`) was removed. It returned the node
instance itself, so `lock (node.DataLock)` was `lock (node)`: one lock shared
between the stack, the node and every caller, taken in an order none of them
could see.

```csharp
// was
lock (node.DataLock)
{
    value = node.Value;
}

// now - the node guards its own state
value = node.Value;
```

If the surrounding operation has to stay atomic across several calls, take a
lock the calling component owns. Do not reach for one that is reachable from a
shared node. Analyzer `UA0025` flags the removed member.

## Migrating code that used BaseVariableValue.Lock

`BaseVariableValue.Lock` was removed, and the constructor now takes a
`System.Threading.Lock` instead of an `object`.

A derived value class - which is what the source generator emits for every
structure variable - synchronizes through the protected `EnterLock()` /
`ExitLock()` pair:

```csharp
EnterLock();
try
{
    // read or write the value fields
}
finally
{
    ExitLock();
}
```

A component that has to make its own state atomic with the value passes a lock
it already owns to the constructor and takes that one directly. This is how the
server keeps its status and its diagnostics mutually exclusive:

```csharp
private readonly Lock m_diagnosticsLock = new();

// the value is constructed with the lock its owner already holds elsewhere
m_status = new ServerStatusValue(statusNode, status, m_diagnosticsLock);

// so the owner synchronizes against the value without the value handing anything out
lock (m_diagnosticsLock)
{
    ...
}
```

Analyzer `UA0026` flags the removed member. Note that regenerating the model
sources with the 2.0 generator produces the `EnterLock()` / `ExitLock()` form
already, so this only affects hand-written derived value classes and callers.

## Migrating code that locked on a NodeState or a NodeBrowser

Remove external `lock (node)` statements: `NodeState` synchronizes its own
attributes and collections. Replace reference check-then-add pairs with
`AddReferenceIfMissing`. Browsers are single-consumer, and `NodeBrowser.DataLock`
is removed (analyzer `UA0027`).

The [node-state migration guide](migrate/2.0.x/node-states.md#nodestate-guards-itself-nodebrowser-is-single-consumer-ua0027)
contains the before/after examples and the `PopulateBrowserSynchronized`
requirement for custom browsers. For I/O-backed iteration, see
[the async iteration seam](migrate/2.0.x/node-states.md#nodebrowser-gains-an-async-iteration-seam).
The current [threading contract](NodeManagers.md#threading-contract-for-nodes-and-browsers)
defines snapshot and synchronization boundaries.

## Migrating code that used ApplicationConfiguration.PropertiesLock

`ApplicationConfiguration.PropertiesLock` was removed. It returned the
properties dictionary itself, so `lock (configuration.PropertiesLock)` was
`lock (configuration.Properties)`: the lock was the data it guarded, shared
between the configuration and every caller, taken in an order none of them could
see.

`Properties` is now a concurrent dictionary, so each individual operation is
already atomic and most callers simply drop the `lock`:

```csharp
// was
lock (configuration.PropertiesLock)
{
    configuration.Properties["MyKey"] = value;
}

// now
configuration.Properties["MyKey"] = value;
```

The one combination that needs more than a single operation is get-or-add, which
has its own member:

```csharp
// was
lock (configuration.PropertiesLock)
{
    if (configuration.Properties.TryGetValue("MyKey", out object? existing))
    {
        return (MyType)existing;
    }
    var created = Build();
    configuration.Properties["MyKey"] = created;
    return created;
}

// now
return configuration.GetOrAddProperty("MyKey", Build);
```

`GetOrAddProperty` deliberately does **not** invoke the factory under a lock, so
a caller cannot hold a critical section across a callback. Under contention the
factory may run more than once; only one result is published and every caller
receives that same instance, so keep the factory free of side effects that would
matter if it ran twice.

One behaviour change is worth knowing: enumerating `Properties` while another
thread writes no longer throws `InvalidOperationException`. It yields a
moment-in-time view instead. Code that relied on the exception to detect
concurrent modification has to detect it some other way.

Analyzer `UA0028` flags the removed member.

## Migrating node types that override FindChild or CreateChild
`NodeState.FindChild` and `NodeState.CreateChild` take
`assignInstanceNodeIds` as their last parameter, and the four argument
`FindChild` / two argument `CreateChild` virtuals are gone. The parameter
defaults to `true`, so **call sites are unaffected**; an override fails to
compile (`CS0115`) until the parameter is added and passed on.

Behaviour note: a node copy — `NodeState.Create(context, source)` and the
`Initialize(ISystemContext, NodeState)` path behind it — now passes
`assignInstanceNodeIds: false`. It no longer asks
`ISystemContext.NodeIdFactory` for identifiers that the copy overwrites
from the source on the very next statement. If your `INodeIdFactory`
counts, reserves or audits every allocation, expect **fewer** calls than in
1.5.378 for the same address space; the resulting NodeIds are unchanged.
Any `NodeState` subclass you own must thread the argument into its
`CreateOrReplace<Child>` calls to get that benefit.

See
[Node states § FindChild and CreateChild](migrate/2.0.x/node-states.md#nodestate-findchild-and-createchild-state-nodeid-assignment)
for the before/after and
[Custom node types and assignment control](NodeManagers.md#custom-node-types-and-assignment-control)
for the runtime rules.

## Adopting replica-consistent NodeIds

When moving a custom 1.5.378 redundant deployment to the 2.0 shared address-space
modules, configure [`UseReplicaNodeIdentity`](ReplicaNodeIdentity.md) with the same
ordered model/instance namespace list on every replica. Namespace 1 remains local
to each ApplicationUri; shared namespaces start at index 2. Do not assume that
previously clock-seeded counters or namespace registration order form a portable
identity contract. Preserve existing agreed IDs explicitly where compatible, or
plan an offline namespace/identifier migration and client cutover.

Unbound legacy shared state is not automatically adopted or renumbered. The
identity module refuses unknown or conflicting stored contracts; see its
new-store provisioning and migration limits before attaching an existing store.

## Removed members on ISession

`ISession.SessionDiagnostics` is removed. It handed out the whole mutable
`SessionDiagnosticsDataType` — the structure the session's diagnostics lock
protects — so a caller could read a field while the owner was writing it.

Every server-side reader wanted one value out of it, and those two values are
now on the interface directly:

```csharp
// was
string? uri = session.SessionDiagnostics?.ClientDescription?.ApplicationUri;
string name = session.SessionDiagnostics?.SessionName ?? string.Empty;

// now
string? uri = session.ClientApplicationUri;
string name = session.SessionName;
```

For anything else in the structure, project it inside `ReadDiagnostics`, which
holds the lock for the duration of the projection:

```csharp
uint reads = session.ReadDiagnostics(diagnostics => diagnostics.ReadCount.TotalCount);
```

`SessionName` is read from the field it was always a copy of rather than from
the diagnostics, because it is assigned once during construction and a value
that cannot change should not cost a lock.

The concrete `Session` still exposes `SessionDiagnostics`; only the interface
loses it.

`ISession.ValidateBeforeActivate` — the synchronous overload with
`out IUserIdentityTokenHandler?` and `out UserTokenPolicy?` parameters — is
removed. It had no caller anywhere in the stack, its samples or its tests other
than tests written for it, and it had been `[Obsolete]` since 1.5.378.

Use `ValidateBeforeActivateAsync`, which returns the same two values as a tuple:

```csharp
(IUserIdentityTokenHandler identityToken, UserTokenPolicy? userTokenPolicy) =
    await session.ValidateBeforeActivateAsync(
        context, clientSignature, userIdentityToken, userTokenSignature, ct)
    .ConfigureAwait(false);
```

The synchronous overload could not verify a user token that required
decryption, so on a secure endpoint it failed closed and directed callers to
the asynchronous path anyway.

The history continuation points moved off `ISession` onto
`ISession.ContinuationPoints`, and no longer pass `object`. `SaveHistory` and
`RestoreHistory` use `IHistoryContinuationPoint`, which carries the point's own
`Guid Id` and extends `IDisposable`:

```csharp
// was
session.SaveHistoryContinuationPoint(state.Id, state);
object? restored = session.RestoreHistoryContinuationPoint(bytes);

// now
session.ContinuationPoints.SaveHistory(state);   // the point carries its Id
IHistoryContinuationPoint? restored = session.ContinuationPoints.RestoreHistory(bytes);
```

Implement `IHistoryContinuationPoint` on whatever type you store. The session
previously disposed only those points that happened to implement `IDisposable`
and silently leaked the rest; every point is now disposed.

## Awaiting custom node-manager cleanup

When directly owning a `CustomNodeManager2`, use `await using` or await
`DisposeAsync()` if subsequent work depends on its resources being released.
Unlike the synchronous cleanup in 1.5.x, `Dispose()` now closes admission and
can return while previously admitted operations finish. The monitored-item
manager and address-space nodes remain alive until those operations return.
New service and lifecycle calls after admission closes throw
`ObjectDisposedException`.

An admitted callback may call `Dispose()` to initiate shutdown, but must not
await its own drain with `DisposeAsync()`. Await completion outside the
callback. Server and master-node-manager asynchronous teardown already await
adapted synchronous managers, so server-owned managers need no additional
disposal call.

## Migrating code that called IServerInternal.Set* mutators

`IServerInternal` no longer exposes the twelve `Set*` binding methods or
`CreateServerObjectAsync`. They were startup plumbing: `StandardServer` calls
each exactly once, in one block, to carry a `Create*` factory result into the
datastore. Publishing them on the interface let any holder rewire a running
server, which would leave every component that had already resolved a subsystem
holding the previous instance.

The supported seam is the factory seam, which already existed for every
subsystem here:

| Instead of | Override | Or register in DI |
| --- | --- | --- |
| `SetRoleManager` | `StandardServer.CreateRoleManager` | `IRoleManager` |
| `SetUserManagement` | `StandardServer.CreateUserManagement` (new) | `IUserManagement` |
| `SetMonitoredItemQueueFactory` | `StandardServer.CreateMonitoredItemQueueFactory` | `IMonitoredItemQueueFactory` |
| `SetSubscriptionStore` | `StandardServer.CreateSubscriptionStore` | `ISubscriptionStore` |
| `SetMainNodeManagerFactory` | `StandardServer.CreateMainNodeManagerFactory` | — |
| `SetNodeManager` | `StandardServer.CreateMasterNodeManager` | — |
| `SetSessionManager` | `StandardServer.CreateSessionManager` / `CreateSubscriptionManager` | `ISessionManager`, `ISubscriptionManager` |
| `SetAggregateManager` | `StandardServer.CreateAggregateManagerAsync` | — |
| `SetModellingRulesManager` | `StandardServer.CreateModellingRulesManagerAsync` | — |
| `SetConformanceUnitsManager` | `StandardServer.CreateConformanceUnitsManagerAsync` | — |

`CreateUserManagement` is new in this release, because user management was the
one subsystem with no factory seam. Registering an `IUserManagement` in the
container also switches on the username/password authenticator; override
`CreateUserManagement` if you want the Part 18 §5 model without that.

`SetIdentityRegistry` is **removed with no replacement**. Nothing ever called
it: the supported route has always been
`ServerIdentityRegistryExtensions.RegisterDefaultAuthenticators`, which adds
authenticators to the default registry rather than replacing it.

The methods remain on the concrete `ServerInternalData`, so code that already
held that type keeps compiling. Binding is now refused once the server has
finished starting — a late `Set*` throws `ServiceResultException` with
`BadInvalidState` naming the operation.

## Migrating IServerStartupTask implementations to IServerContext

`IServerStartupTask.OnServerStartedAsync` now receives an `IServerContext`
instead of an `IServerInternal`. `IServerInternal` derives from
`IServerContext`, so the host still passes the same object; only the
declared parameter type changes and an implementation fails to compile
(`CS0535`) until its signature is updated.

`IServerContext` is the ambient view of a running server. It carries what
is genuinely server-wide and nothing else — it deliberately does not hand
out the server's subsystems. A startup task that needs a subsystem takes it
as a constructor dependency, which every implementation in this repository
already did for its other dependencies.

Rewrite the member reads that no longer resolve:

| Was | Now |
| --- | --- |
| `server.Telemetry` | `server.DefaultSystemContext.Telemetry` |
| `server.NamespaceUris` | `server.DefaultSystemContext.NamespaceUris` |
| `server.ServerUris` | `server.DefaultSystemContext.ServerUris` |
| `server.TypeTree` | `server.DefaultSystemContext.TypeTable` |
| `server.Factory` | `server.DefaultSystemContext.EncodeableFactory` |
| `server.DiagnosticsNodeManager.FindPredefinedNode<T>(id)` | `server.FindPredefinedNode<T>(id)` |
| `server.NodeManager.NodeManagers` + a type test | `server.FindNodeManagers<TCapability>()` |
| `server.SessionManager`, `server.SubscriptionManager`, `server.RequestManager`, `server.AggregateManager`, `server.RoleManager`, `server.IdentityRegistry`, … | constructor injection |

`server.MessageContext` is unchanged and remains on `IServerContext`. Do
**not** substitute `server.DefaultSystemContext.AsMessageContext()` for it:
that conversion produces a context with *default* decoding limits rather
than the server's configured `MaxStringLength`, `MaxArrayLength` and
`MaxByteStringLength`, which silently widens what your component accepts.

Tests that hand a `Mock<IServerInternal>` to a startup task keep compiling,
because the mock still satisfies `IServerContext`. Stub the members the task
actually reads now — typically `DefaultSystemContext` and any
`FindNodeManagers<T>()` — or the mock returns `null` and the task fails at
run time rather than at build time.

## Removed members on IServerInternal

The following members had no consumer anywhere in the stack, its samples or
its tests, and have been removed. Each has a direct replacement:

| Removed | Use instead |
| --- | --- |
| `CloseSession(OperationContext, NodeId, bool)` | `CloseSessionAsync(…)` |
| `Status` | `CurrentState` to read, `UpdateServerStatus` to write |
| `ServerDiagnostics` | `UpdateServerDiagnostics(Action<…>)` |
| `DiagnosticsEnabled` | `IDiagnosticsNodeManager.DiagnosticsEnabled` |
| `ModellingRulesManager`, `ConformanceUnitsManager` | the concrete `ServerInternalData`, which owns them |

`MessageContext`, `DefaultSystemContext`, `CurrentState`, `ServerObject`,
`ReportEventAsync`, `CloseSessionAsync`, `DeleteSubscriptionAsync` and
`UpdateServerDiagnostics` all moved *down* to `IServerContext`, the ambient
view of a running server that `IServerInternal` now derives from. They remain
reachable through `IServerInternal` unchanged, so no call site has to move.
`CurrentState` is read-only on the ambient interface; the server itself still
sets it.

## Migrating servers that relied on unserved history advertisement

Server startup now reconciles variables that advertise history with the
historian providers actually wired into the server. If a variable has
`Historizing=true` or `HistoryRead` / `HistoryWrite` access-level bits
from a NodeSet but no `IHistorianProvider` resolves for it, the server
clears the advertisement and masks the attribute read callbacks before
accepting clients. Variables with a provider keep their NodeSet-declared
history surface.

If a client or CTT setup expected `HistoryRead` solely because the
NodeSet declared it, wire a historian instead of relying on the static
flag: use `builder.UseHistorian()` and `.Historize(...)`, register a
provider through the server-wide historian registry, or override
`GetHistorianProvider(NodeState)` in the node manager. See
[Server address-space metadata](NodeManagers.md#server-address-space-metadata) and
[Historical Access](HistoricalAccess.md).

## Migrating custom ISessionManager implementations to ShutdownAsync

`ISessionManager.Shutdown()` is **gone**, replaced by
`ShutdownAsync(CancellationToken)`. `SessionManager` previously started its
session monitor loop with a discarded `Task.Factory.StartNew(...)`, so
`Shutdown()` only *signalled* the loop and returned: the server could
finish tearing down while the monitor was still closing expired sessions
and raising keep-alive events against half-disposed state. There is no
correct synchronous way to wait for that loop — blocking on it would be
sync-over-async — so the synchronous overload was removed rather than
kept as a trap. `ShutdownAsync` cancels the loop and awaits it before
disposing the sessions, matching `ISubscriptionManager.ShutdownAsync`.

**Callers** await instead of calling:

```csharp
// before
server.SessionManager.Shutdown();

// after
await server.SessionManager.ShutdownAsync(cancellationToken)
    .ConfigureAwait(false);
```

**Implementers** of `ISessionManager` (for example a manager registered
through `services.AddSessionManager<T>()`) replace `Shutdown` with
`ShutdownAsync`. If your implementation has no background work, return a
completed task:

```csharp
public ValueTask ShutdownAsync(CancellationToken cancellationToken = default)
{
    CloseAllSessions();
    return default;
}
```

Deriving from `SessionManager` requires no change beyond renaming any
`Shutdown` override: `ShutdownAsync` is `virtual` and the base
implementation already awaits the monitor loop.

## Configuring distributed address-space storage

Direct `InMemoryNodeStateStore` writers require their backend to implement
`ISharedKeyValueStoreConsistency` and provide linearizable operations for
`election/addressspace-sequence`. A bare CRDT store is not a valid writer
backend. Compose it with the shared Raft coordinator using
`HybridSharedKeyValueStore`, or use the fluent consistency registration before
`UseDistributedAddressSpace`.

Use strong state storage for authoritative bootstrap and compacted snapshots.
CRDT payload storage retains merge-only hydration and delta replay without
absence-based cleanup or snapshot compaction. These are configuration
requirements of the distributed-state extension, not a 1.5 persisted-format
migration guarantee. See
[address-space consistency](HighAvailability.md#activepassive-address-space-consistency).

## Migrating SamplingGroupManager create/modify overrides

The public virtual `SamplingGroupManager.CreateMonitoredItem` and
`ModifyMonitoredItem` entry points, and the protected virtual creation
factory, have been removed. They did not accept a separate server-revised
filter, so preserving their dispatch required request-local state merely to
carry that filter through existing overrides.

Use the node manager's `IMonitoredItemManager` pipeline for creation and
modification. For custom item construction, return
`MonitoredItemCreateDecision.Use(factory)` from `OnCreateMonitoredItem` or
`AsyncCustomNodeManager.OnCreatingMonitoredItemAsync`; the stack registers
and owns the returned item. See
[monitored-item creation and lifecycle](NodeManagers.md#monitored-item-creation-and-lifecycle).

Sampling-group creation/modification now receives the original request and
revised filter directly through internal, nonvirtual methods. The request is
not rewritten, and the unrelated monitoring/lifecycle hooks are unchanged.
This is an intentional API removal, not an obsolete forwarding shim.

## Migrating callers of the synchronous MonitoredNode2 notification wrappers

`MonitoredNode2.OnReportEvent` and `MonitoredNode2.OnMonitoredNodeChanged`
are `[Obsolete]`; use `OnReportEventAsync` and
`OnMonitoredNodeChangedAsync`. Nothing in the stack wires the synchronous
pair any more — notifiers are attached through
`NodeState.OnReportEventAsync` and `NodeState.OnStateChangedAsync` — and
both wrappers block the calling thread whenever the bounded notification
channel is full, or whenever the node has an asynchronous read handler.
Blocking there occupies a thread while waiting for a consumer that needs
a thread of its own, which starves the thread pool under load.

```csharp
// before
monitoredNode.OnReportEvent(context, node, e);

// after
await monitoredNode.OnReportEventAsync(context, node, e, cancellationToken)
    .ConfigureAwait(false);
```

The wrappers still work and are unchanged in behaviour, so this is a
warning to act on rather than a break.

<a id="ua0029"></a>

## Migrating callers of the SecurityPolicies lookup and cryptography statics

The static lookup and cryptography methods on `SecurityPolicies` have
**moved** to `ISecurityPolicyRegistry`. They read the set of registered
security policies, so they are members of the registry that owns that set
rather than free functions on a constants class.

Moved: `GetUri`, `GetDisplayName`, `GetDisplayNames`,
`IsValidSecurityPolicyUri`, `GetDefaultUris`, `GetDefaultEccUris`,
`GetDefaultDeprecatedUris`, `Encrypt` and `Decrypt`.

**The policy URI constants are unaffected.** `SecurityPolicies.None`,
`SecurityPolicies.Basic256Sha256` and the rest stay exactly where they are,
which is the overwhelming majority of references to this type.

Resolve an `ISecurityPolicyRegistry` where a container is in scope, so the
policies that application registered are the ones used:

```csharp
// before
string uri = SecurityPolicies.GetUri("Basic256Sha256");

// after - the application's own policy set
public sealed class MyService(ISecurityPolicyRegistry policies)
{
    public string? Uri => policies.GetUri("Basic256Sha256");
}
```

Where there is no container — configuration loading, for instance — use
the fallback, which carries the built-in policies:

```csharp
string? uri = SecurityPolicies.Default.GetUri("Basic256Sha256");
```

`Encrypt` and `Decrypt` additionally **lose their `ILogger` argument**. The
registry is created with an `ITelemetryContext` and reports through the
logger it made from it:

```csharp
// before
EncryptedData data = SecurityPolicies.Encrypt(certificate, uri, plainText, logger);

// after
EncryptedData data = policies.Encrypt(certificate, uri, plainText);
```

Registering a policy through the container no longer changes what other
code in the same process sees. `AddSecurityPolicy` applies the policy to
the registry that container owns, so two applications hosted together keep
separate policy sets. Use `AddSecurityPolicyRegistry()` to resolve a
registry without contributing a policy of your own.

The `OPCFoundation.NetStandard.Opc.Ua.MigrationAnalyzer` package restores
the removed members as `[Obsolete]` extension members that forward to
`SecurityPolicies.Default`, so a 1.05.378 application compiles with
a warning rather than an error. The `ILogger` argument on the `Encrypt` and
`Decrypt` shims is accepted and **ignored**.

<a id="ua0030"></a>

## Migrating code that drove the server subscription publish pipeline

The server publish pipeline is internal. Analyzer `UA0030` identifies removed
`Opc.Ua.Server.ISubscription` members: remove no-op calls and use the service
operations for publishing, acknowledgements, transfers, and session teardown.
Custom server subscriptions must derive from `Subscription`.

See the [server subscription migration](migrate/2.0.x/sessions-subscriptions.md#opcuaserverisubscription-the-publish-pipeline-is-server-internal)
for the complete removed-member list, replacement service paths, and custom
implementation requirements. `SessionPublishQueue` is internal too; applications
must not drive it directly.

The [no-obsolete-shim rule](#why-there-is-no-obsolete-shim) applies because
restoring members to a public interface would break downstream implementers.

## Migrating channel subclasses that guarded state with DataLock

`UaSCBinaryChannel.DataLock` has been **removed**. The channel no longer
serialises any of its own state on it, so taking it excluded nothing.

A monitor cannot be held across an `await`, and the secure channel open
path has to be able to await once a private key may be served over a
network — see [Crypto provider](CryptoProvider.md). The channel now uses
an internal gate that can be entered from a synchronous or an
asynchronous path. Unlike a monitor the gate is **not re-entrant**: every
channel path that used to take the lock while already holding it now
calls a lock-free `Core` variant instead.

That has one consequence for subclasses. The channel calls
`HandleSocketError`, `NotifyMonitors` and `CompleteReverseHello` from
paths that may already hold the gate, so an override of any of them must
not call back into a channel method that takes it — `ForceChannelFault`
and `SendResponse` in particular. Such an override would have silently
nested before and will now block.

For the same reason `SaveIntermediateChunk`, `GetSavedChunks` and
`DoMessageLimitsExceeded` take an additional `gateHeld` argument. It says
whether the calling frame already holds the gate, so that an override
which tears the channel down can pick the locking or the lock-free path:

```csharp
// before
protected override void DoMessageLimitsExceeded()
{
    base.DoMessageLimitsExceeded();
    Shutdown(new ServiceResult(StatusCodes.BadResponseTooLarge));
}

// after
protected override void DoMessageLimitsExceeded(bool gateHeld)
{
    base.DoMessageLimitsExceeded(gateHeld);

    if (gateHeld)
    {
        ShutdownCore(new ServiceResult(StatusCodes.BadResponseTooLarge));
        return;
    }

    Shutdown(new ServiceResult(StatusCodes.BadResponseTooLarge));
}
```

There is no drop-in replacement to offer across an assembly boundary,
because the gate has to be entered asynchronously on the open path and
its correctness depends on rules that only hold inside the channel
implementation. A subclass outside this stack that guarded **its own**
state with `DataLock` should introduce its own synchronisation:

```csharp
// before
lock (DataLock)
{
    m_myState = value;
}

// after
private readonly System.Threading.Lock m_myLock = new();

using (m_myLock.EnterScope())
{
    m_myState = value;
}
```

A subclass that took `DataLock` in order to be mutually exclusive with
the **channel's** state transitions was already relying on an
implementation detail, and can no longer do so.

## Transport resource limits

Applications migrating from 1.5.x have a server-wide budget for retained
intermediate-message buffers. With the reference server's 4 MiB maximum message
size, the default budget is **64 MiB**. Under SharedOnly, channels without an
activated Session may retain chunks only while total usage remains within the
lower **32 MiB**. Balanced instead divides the same total between shared memory
and memory reserved for specific kinds of work. With a 65,536-byte buffer limit,
16.25 MiB is reserved for verified startup traffic and 16.25 MiB for continuity
or reconnect traffic, leaving 31.5 MiB shared. Other traffic cannot borrow these
reserves. Channels already carrying an activated Session can use the continuity
reserve for incomplete messages. A chunk that does not fit discards its partial
message and closes the channel with `BadTcpNotEnoughResources`. Final chunks,
single-chunk requests, response buffers, and client buffers are not charged to
this reassembly budget. Once a complete request has been decoded, separate
limits control how many requests may wait, execute, or remain parked, and how
much request data they may retain. A request refused at that stage receives
`BadServerTooBusy`, even if it fitted in a single transport chunk.

For workloads with many simultaneous large requests, set
`WithChunkReassemblyBudget(maxBytes)` on the Dependency Injection (DI) server builder or assign
`ServerBase.ChunkReassemblyBudget` before startup. A host opening listeners
directly can share a budget through `TransportListenerSettings.ChunkReassemblyBudget`.
See [incomplete messages](RateLimiting.md#incomplete-messages) for sizing and
sessionless configuration. General buffer-manager limits remain opt-in.

Server-channel `ChannelLifetime` also bounds an unfinished message from its
first retained chunk, even if more chunks keep arriving. Size this lifetime
for legitimate large transfers without relying on continuation chunks to
extend it indefinitely. A zero or negative value uses the 30-second default
for message assembly. It does not disable assembly cleanup. See
[incomplete-message limits](Transports.md#incomplete-message-resource-limits).

## Migrating channel subclasses that override HandleIncomingMessage

`UaSCBinaryChannel.HandleIncomingMessage` and `OnChunkReceived` have been
**removed**, as has the `protected WriteAsymmetricMessage` overload that
returned the signature through an `out` parameter. The receive loop calls
`HandleIncomingMessageAsync` and `OnChunkReceivedAsync`, so that the
secure channel open path can await a private key served over a network —
see [Crypto provider](CryptoProvider.md).

A synchronous override cannot be kept working underneath the
asynchronous path without defeating the point of it, so **an existing
override must be moved**. The signature gains a `CancellationToken` and
returns `ValueTask<bool>`:

```csharp
// before
protected override bool HandleIncomingMessage(
    uint messageType, ArraySegment<byte> messageChunk)
{
    ...
}

// after
protected override async ValueTask<bool> HandleIncomingMessageAsync(
    uint messageType, ArraySegment<byte> messageChunk, CancellationToken ct)
{
    ...
}
```

An override that has nothing to await can return a completed value
without going asynchronous at all:

```csharp
protected override ValueTask<bool> HandleIncomingMessageAsync(
    uint messageType, ArraySegment<byte> messageChunk, CancellationToken ct)
{
    return new ValueTask<bool>(HandleSynchronously(messageType, messageChunk));
}
```

The buffer-ownership contract is unchanged: return `true` when the
implementor takes ownership of the chunk, and it will not be returned to
the buffer manager for you.

`ReadAsymmetricMessageAsync` and `WriteAsymmetricMessageAsync` return
`AsymmetricMessage` and `AsymmetricWriteResult` rather than using `out`
parameters, which an asynchronous method cannot have. Use
`WriteAsymmetricMessageAsync` in place of the removed synchronous
overload.

## Migrating custom IUserDatabase implementations

`IUserDatabase` gained four members. A store now persists user metadata
alongside credentials, so disabled and `MustChangePassword` decisions survive a
restart and an administrative password reset is a single transaction rather than
a delete followed by a create:

```csharp
IReadOnlyList<UserManagementDataType> GetUsers();

bool CreateUser(
    string userName,
    ReadOnlySpan<byte> password,
    ArrayOf<Role> roles,
    UserConfigurationMask userConfiguration,
    string description);

bool ResetPassword(
    string userName,
    ReadOnlySpan<byte> newPassword,
    UserConfigurationMask userConfiguration,
    string description);

bool UpdateUserMetadata(
    string userName,
    UserConfigurationMask userConfiguration,
    string description);
```

The five 1.5.378 members are unchanged. Implement the new ones on your store, or
derive from `LinqUserDatabase` / `JsonUserDatabase`, which provide in-memory and
atomic file-backed transactions respectively.

Each mutation must commit as one transaction, and a rejected or failed write must
leave both the live and the persisted record unchanged. `ChangePassword` must
clear `UserConfigurationMask.MustChangePassword` in the same transaction as the
password it commits. `ResetPassword` must preserve the user's identity and roles
and must not delete and recreate the user.

There is no optional-capability fallback: `UserManagement` requires these members
and no longer keeps metadata only in memory, so a store that cannot persist
metadata should reject the write by returning `false` rather than silently
accepting it.

## Reading certificates from SecureChannelContext

`SecureChannelContext.ClientChannelCertificate`, `ServerChannelCertificate` and
`ChannelThumbprint` are `ByteString` instead of `byte[]`. The transport now shares
one buffer of a channel's certificates with every request on that channel instead
of copying the DER encoding per request, and a `ByteString` is a read-only view of
it, so a request handler or an `IResourceIsolationClassifier` cannot alter the
evidence later requests are checked against.

The constructor still takes `byte[]` and wraps it without copying. A missing
value is `IsNull`; an empty array stays empty, not null.

| Before | After |
|---|---|
| `context.ClientChannelCertificate == null` | `context.ClientChannelCertificate.IsNull` |
| `context.ClientChannelCertificate?.Length > 0` | `context.ClientChannelCertificate.Length > 0` |
| `context.ClientChannelCertificate.AsSpan()` | `context.ClientChannelCertificate.Span` |
| `Certificate.FromRawData(context.ClientChannelCertificate)` | `Certificate.FromRawData(context.ClientChannelCertificate.Memory)` |
| `context.ClientChannelCertificate.ToByteString()` | `context.ClientChannelCertificate` |
| passing the array to an API that takes `byte[]?` | `value.IsNull ? null : value.ToArray()` |

Keep a missing certificate null when converting: the session signature helpers
hash an empty array but skip a null one, so substituting `ToArray()` for a null
value changes the signature data.

## ContentFilter NULL semantics follow OPC 10000-4 1.05.07

`FilterEvaluator` applies the NULL rules of
[OPC 10000-4 §7.7.3](https://reference.opcfoundation.org/Core/Part4/v105/docs/7.7.3)
to event where-clauses and every other `ContentFilter`:

- An element with a null operand evaluates to NULL (except `IsNull`), and a
  filter that ends as NULL is FALSE. `Equals(field, 0)` no longer matches an
  event without that field, and `Not(Equals(field, 5))` no longer matches it
  either.
- `IsNull` is TRUE for the null value of a nullable built-in type (a null
  String, ByteString, NodeId, the all-zero Guid, `DateTime.MinValue`, …) and for
  a null or empty array, which
  [OPC 10000-6 §5.1.11](https://reference.opcfoundation.org/Core/Part6/v105/docs/5.1.11)
  treats as the same. A zero, `false` or a Good StatusCode is a value.
- Operands that cannot be converted to a common type make `Between` FALSE
  instead of NULL.

Clients whose where-clauses relied on the old matching of missing fields should
test them explicitly with `IsNull`, for example `Or(IsNull(field), Equals(field, 0))`.

## Write service Value semantics follow OPC 10000-3 and OPC 10000-4

A Value written through the Write service is now checked the same way on every path of
`BaseVariableState` ([OPC 10000-4 §5.11.4](https://reference.opcfoundation.org/Core/Part4/v105/docs/5.11.4)):

- **StatusWrite / TimestampWrite.** Without the `StatusWrite` bit of the AccessLevel only the
  StatusCode Good may be written, and without the `TimestampWrite` bit only a null
  SourceTimestamp ([OPC 10000-3 §8.57](https://reference.opcfoundation.org/Core/Part3/v105/docs/8.57)).
  Other combinations return `Bad_WriteNotSupported`. The bits must also be present in the
  effective UserAccessLevel of the caller (including `OnReadUserAccessLevel`); otherwise the
  write returns `Bad_UserAccessDenied`. "Good" is compared by the code bits only, so Good
  with info bits (for example SemanticsChanged or LimitBits) needs no StatusWrite, while a
  Good SubCode such as `Good_Clamped` does. Server code that assigns `Value`,
  `StatusCode` or `Timestamp` directly is not affected. Set
  `AccessLevels.StatusWrite | AccessLevels.TimestampWrite` on both the AccessLevel and the
  UserAccessLevel of Variables whose clients legitimately write status codes or source
  timestamps.
- **Type check before `OnWriteValue`.** The DataType and ValueRank are verified before the
  synchronous `OnWriteValue` handler (fluent `OnWrite`) runs, so a handler no longer
  receives a value of the wrong type; the write returns `Bad_TypeMismatch`.
- **IndexRange with `OnWriteValue`.** After the handler accepts an IndexRange write, the slice
  is merged into the cached value instead of replacing it.
- **Enumerations.** Writing an Int32 that is not a defined value of an Enumeration DataType
  registered with the encodeable factory returns `Bad_OutOfRange` (also for every element of
  an array or matrix).
- **OptionSet.** A written OptionSet structure must have Value and ValidBits of the same size
  as the stored value and may only select valid bits, otherwise `Bad_OutOfRange`. The valid
  bits are the ValidBits of the stored value (an all-zero mask means no bit is valid), or the
  bits of the OptionSet DataType definition when the stored value has no ValidBits. The selected
  bits are merged into the stored value
  ([OPC 10000-3 §8.40](https://reference.opcfoundation.org/Core/Part3/v105/docs/8.40)).
  The same applies to each element of an OptionSet array or matrix, including elements
  written through an IndexRange.

Server code that changes a Property with semantic meaning (for example `EURange` or
`EngineeringUnits`) directly calls `ReportPropertyValueChanged(context, property, previousValue)`
or `ReportSemanticChange(context, property)` on its `AsyncCustomNodeManager` or
`CustomNodeManager2`, so that a SemanticChangeEvent is raised and the next value notification
carries the SemanticsChanged bit ([OPC 10000-3 §5.6.2](https://reference.opcfoundation.org/Core/Part3/v105/docs/5.6.2)).

The PubSub subscriber that writes TargetVariables of an external server retries a write that
fails with `Bad_WriteNotSupported` without the received timestamps and remembers the target
([OPC 10000-14 §6.2.11.1](https://reference.opcfoundation.org/Core/Part14/v105/docs/6.2.11.1)).

## Address space permissions, Call, NodeManagement and locales follow OPC 10000-3

- **Role permissions on type nodes.** ObjectTypes, VariableTypes and their children stay
  browsable and readable for every user, but all other permission bits are now checked on
  them ([OPC 10000-3 §8.55](https://reference.opcfoundation.org/Core/Part3/v105/docs/8.55)).
  In particular `ReceiveEvents` is verified on the EventType: the standard NodeSet grants it on
  `AuditEventType` and its subtypes only to `SecurityAdmin`, so Anonymous and
  AuthenticatedUser sessions no longer receive audit events. Give the users that monitor
  audit events the SecurityAdmin role (or change the RolePermissions of the audit event types).
  `MasterNodeManager.ValidateAccessRestrictions` has an overload that takes the requested
  permission; the existing two-argument overload behaves as a Browse/Read request.
- **Call.** The `Call` permission is required on the Object passed as `objectId` as well as on
  the Method, and the Object's AccessRestrictions apply. When the `methodId` is the Method
  declaration of the ObjectType, the Method of the Object with the same BrowseName is
  permission-checked and invoked ([OPC 10000-4 §5.12.2.2](https://reference.opcfoundation.org/Core/Part4/v105/docs/5.12.2.2));
  bind method handlers on the instance Methods. Trailing input arguments described by
  `HasOptionalInputArgumentDescription` may be omitted by the client and reach the handler as
  `Variant.Null`; override `MethodState.GetOptionalInputArgumentCount` to customize this.
- **NodeManagement.** AddNodes rejects abstract TypeDefinitions (including `BaseVariableType`,
  `BaseEventType` and Interfaces) with `Bad_TypeDefinitionInvalid` and creates the Mandatory
  InstanceDeclarations of the type below the new node. AddReferences, DeleteReferences and
  DeleteNodes bump the `NodeVersion` and raise `GeneralModelChangeEvent`s.
- **Locales.** Read (DisplayName, Description, InverseName, LocalizedText values), Browse and
  data change notifications return the translation that best matches the session's LocaleIds,
  or all translations for `mul`
  ([OPC 10000-4 §5.4](https://reference.opcfoundation.org/Core/Part4/v105/docs/5.4)). The
  selection uses `ResourceManager.TranslateValue`, which calls the overridable
  `ResourceManager.Translate`. Locale ids are matched case-insensitively.
- **NodeSet import.** `UANodeSet.Import` completes the `StructureDefinition` of structure
  subtypes with the fields of their base types (base fields first,
  [OPC 10000-3 §8.48](https://reference.opcfoundation.org/Core/Part3/v105/docs/8.48)), creates a
  definition for structures without fields, and sets the `DefaultEncodingId` of concrete
  structures. `StructureDefinition.FirstExplicitFieldIndex` marks the first field of the
  subtype; NodeSet export writes only those fields.

## Migrating from 1.05.377 to 1.05.378

### Asynchronous as default

The server now supports `AsyncNodeManagers`; see
[Server Async (TAP) Support](AsyncServerSupport.md). The client APIs are
async by default and all synchronous and APM-based API has been
deprecated. To migrate, update your code to use the `Async` version of
every API where possible. Not recommended but for expedience you can
call the `Async` version synchronously with
`GetAwaiter().GetResult()`.

### Observability

Observability is now plumbed through `ITelemetryContext`. The legacy
static `Utils.SetLogger` / `Utils.Trace*` model has been removed in
2.0. See
[`migrate/2.0.x/telemetry.md`](migrate/2.0.x/telemetry.md) for OLD
vs NEW snippets, the per-type constructor matrix, and the full
inventory of removed / `[Obsolete]` `Utils` APIs.

Configuration-level trace apply APIs were removed as well: `TraceConfiguration.ApplySettings()` and `ApplicationConfigurationBuilder` trace setters (`SetOutputFilePath`, `SetDeleteOnLoad`, `SetTraceMasks`). Configure logging through `ITelemetryContext` instead.
## Migrating from 1.04 to 1.05

A few features are still missing to fully comply with 1.05, but
certification for v1.04 is still possible with the 1.05 release.

## Support

For additional migration support:

- Review sample applications in the repository.
- Check unit tests for usage patterns.
- Use the
  [`OPCFoundation.NetStandard.Opc.Ua.MigrationAnalyzer`](https://www.nuget.org/packages/OPCFoundation.NetStandard.Opc.Ua.MigrationAnalyzer)
  package — 26 implemented analyzer rules through `UA0030` (excluding
  `UA0013`, `UA0016`, `UA0017`, and the shim-only `UA0029`) map across the
  [`migrate/2.0.x/`](migrate/2.0.x/README.md) guides and the cross-cutting
  notes on this page; 14 rules apply safe edits via a code-fixer. `UA0029`
  is currently a runtime-shim/manual marker surfaced through `CS0618`, not an
  analyzer diagnostic.
- Open an issue on
  [OPCFoundation/UA-.NETStandard](https://github.com/OPCFoundation/UA-.NETStandard/issues).