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Multitenancy With the Thread Context

What Is a Thread Context?

The SAP Cloud SDK for Java provides a so-called ThreadContext. It serves as thread-safe storage for potentially sensitive information. Specifically, the following three objects are stored:

  • The current Tenant
  • The current Principal (User)
  • The JSON Web Token (JWT)

This information is used throughout the SAP Cloud SDK to provide features like tenant and principal isolation, JWT verification and authorization against other systems and services. To ensure different tenants and users are properly isolated in an application, this information is always limited to the thread it was created on unless it is explicitly passed on by the application (see Propagating the Thread Context).

info

Multi-tenancy describes the access of different, technically separated user groups to the same instance of an application. In the terms of XSUAA service, these user groups are called Tenants, while in terms of Identity Authentication Service (IAS) they are called Zones.

The SAP Cloud SDK for Java uses the term Tenant to refer to both XSUAA Tenants and IAS Zones. This implies, that the tenantId exposed in the Tenant interface either returns the tenantId or zoneId, depending on the context you are currently running in.

How Is a Thread Context Created?

The SAP Cloud SDK provides a RequestFilter that will listen to incoming HTTP requests. This filter will extract all provided HTTP headers from the incoming request and store them in the ThreadContext. Additionally, if the Authorization header contains a JWT from the AppRouter, the filter will:

  • Verify this token
  • Store it in the ThreadContext and
  • Pull the Tenant and Principal information from it
Integration with CAP

In case you are using CAP (with the cds-integration-cloud-sdk dependency) the Tenant,Principal information and Headers will instead be derived from the CAP Request Context. That also means that the ThreadContext will not be used when accessing this information. Please note that the RequestScopedHttpClientCache which is the default being used by the SAP Cloud SDK cannot work with this approach.

How Can the Thread Context Be Used?

Accessing Information

The Thread context can be accessed via the static ThreadContextAccessor.

For the frequently needed HTTP Headers, Tenant, Principal, and JWT there are also dedicated accessors:

Storing Information

The ThreadContext allows for some manipulation by the application. However, oftentimes it is more convenient to leverage the executeWith...() functionality offered by the dedicated accessors.

Consider a scenario where some part of the code should run on behalf of a specific tenant. In that case you can override the current tenant explicitly:

TenantAccessor.executeWithTenant(customTenant, () -> doStuff());
CAP Integration

The above does not apply for any accessors like TenantAccessor, PrincipalAccessor, RequestHeaderAccessor when using the CAP framework (with the cds-integration-cloud-sdk dependency). When using CAP the Tenant,Principal information and Headers are derived from the RequestContext. Please refer to this section on how to override existing values in the CAP context.

caution

Be aware, that the executeWith methods shown above only replaces the given property, but does not update properties derived from it.

Example: You have a special AuthToken, that you propagate with AuthTokenAccessor.executeWithAuthToken. Inside the given code block only the AuthToken will be replaced, while e.g. the Tenant is the same as in the original context.

If you want to start a fresh context based on a given AuthToken, for example accessing information of the provider tenant while in a subscriber context, have a look at this code:


Tenant retrieveProviderTenant()
{
// retrieves an access token from the provider context
AuthToken providerXsuaaAccessToken = AuthTokenAccessor.getXsuaaServiceToken();
return new ThreadContextExecutor()
// don't reuse the original context
.withoutParentThreadContext()
// add the provider token into the new context
.withListeners(new AuthTokenThreadContextListener(providerXsuaaAccessToken))
// return the actual provider tenant
.execute(TenantAccessor::getCurrentTenant);
}

Manipulating HTTP Headers

The RequestHeaderAccessor#getHeaderContainer() method provides convenient access to the HTTP headers of the current incoming request. It returns an instance of RequestHeaderContainer, which is, by API contract, an immutable container that allows case insensitive access to HTTP header values. Although the RequestHeaderContainer cannot be altered once created, there are scenarios where manipulating HTTP headers is required. In such cases, a new instance of RequestHeaderContainer can easily be created in a few different ways.

Create a new RequestHeaderContainer From Scratch

A common way to represent HTTP headers is to use Map<String, String> for 1:1 relationships between header names and values or even Map<String, Collection<String>> for 1:n relationships. To make the transition from either of those representations to the SAP Cloud SDK's RequestHeaderContainer as easy as possible, the DefaultRequestHeaderContainer offers corresponding factory methods:

  • fromSingleValueMap(Map<String, String>) and
  • fromMultiValueMap(Map<String, Collection<String>>)

Besides these two factory methods, the DefaultRequestHeaderContainer also offers the possibility to create an instance of RequestHeaderContainer.Builder through the static builder() method. An example for how the returned Builder can be used is shown in the chapter below.

Additionally, to make customizing the current HTTP headers even easier, the RequestHeaderAccessor comes with an overload of the executeWithHeaderContainer method that accepts a Map<String, String>.

Updating an existing RequestHeaderContainer

As pointed out above, the RequestHeaderContainer is an immutable structure. Therefore, updating an already existing instance is not possible.

However, in cases where you would like to, for example, just add a new custom header to the set of existing headers, the RequestHeaderContainer offers the toBuilder method. As the name suggests, this method can be used to retrieve an instance of RequestHeaderContainer.Builder. In contrast to the RequestHeaderContainer, the Builder can be changed as much as needed. Additionally, the toBuilder method will make sure that the returned Builder instance is already pre-filled with all HTTP headers that are also present in the instance of RequestHeaderContainer.

To make things less theoretical, let's examine an example.

Example: Usage of the Builder

Assume your application received an HTTP request that contains the following headers

  • Authorization: Bearer DUMMY_JWT
  • Set-Cookie: cookie-1; cookie-2
  • Accept-Language: en-US
  • x-app-specific-header: customer-value

These values can be accessed as follows:

RequestHeaderContainer headers = RequestHeaderAccessor.getHeaderContainer();
headers.getHeaderValues("authorization"); // will return ["Bearer DUMMY_JWT"]
headers.getHeaderValues("set-cookie"); // will return ["cookie-1", "cookie-2"]
headers.getHeaderValues("accept-language"); // will return ["en-US"]
headers.getHeaderValues("x-app-specific-header"); // will return ["customer-value"]

Note how accessing the values for specific HTTP headers will work independent of the casing of the provided name.

Now let's say your use case requires that HTTP cookies shall not be leaked into further application execution. Additionally, you have to make sure that the x-app-specific-header contains an additional application provided value. Last but not least, your application should always serve German customers and, therefore, you need to make sure the Accept-Language header is always adjusted accordingly.

Luckily, using the Builder API, fulfilling these requirements is straightforward:

RequestHeaderContainer updatedHeaders =
headers
.toBuilder()
.withoutHeader("set-cookie")
.withHeader("x-app-specific-header", "application-value")
.replaceHeader("accept-language", "de-DE")
.build();

Once again, the API guarantees that header names are treated case insensitively.

Finally, to make sure the updated headers are also taken into consideration, you have to overwrite the existing headers in our ThreadContext. This can be done using the following code:

RequestHeaderAccessor.executeWithHeaderContainer(updatedHeaders, () -> yourBusinessLogic());

Running Asynchronous Operations

As the name suggests the ThreadContext is bound to a thread, more specifically to the one it was created. If asynchronous operations need to access the information, it has to be propagated to the new threads.

The following code achieves this:

ThreadContextExecutor executor = new ThreadContextExecutor();
Callable operationWithContext = () -> executor.execute(() -> operation());

invokeAsynchronously(operationWithContext);

Take note that the ThreadContextExecutor is created before performing the asynchronous operation. This is important because only at that time the context is available and will be propagated.

A similar approach can be applied with the RequestHeader, Tenant, Principal and AuthToken accessors. This code runs an asynchronous operation with a dedicated tenant:

Callable operationWithTenant = TenantAccessor.executeWithTenant(customTenant, () -> operation());

invokeAsynchronously(operationWithContext);
CAP Integration

In case you are using CAP the CAP Request Context needs to be passed on instead of the ThreadContext. Please refer to this guide on how to achieve this.

Thread Lifecycle

Be cautious with long-running, asynchronous operations. A propagated thread context will only persist as long as the thread lives that it was created on. So when the parent thread dies the context will cease to exist and no longer be available in any of the threads.

Workaround
You can work around this limitation with the following approach:
RequestHeaderContainer requestHeaders = RequestHeaderAccessor.tryGetHeaderContainer().getOrElse(RequestHeaderContainer.EMPTY);
ThreadContextExecutor executor = new ThreadContextExecutor().withThreadContext(new DefaultThreadContext())
.withListeners(new RequestHeaderThreadContextListener(requestHeaders));

Callable operationWithContext = () -> executor.execute(() -> operation());
invokeAsynchronously(operationWithContext);