Object Store Auditing: Architecture

This the architecture document of the S3A Auditing component.

The S3A auditing subsystem is defined in the package org.apache.hadoop.fs.s3a.audit.

This package is declared LimitedPrivate; some classes inside are explicitly declared @Public (e.g AuditConstants) while others @Private. If declared @Private, external auditing modules MUST NOT use them.

@InterfaceAudience.LimitedPrivate("auditing extensions")
package org.apache.hadoop.fs.s3a.audit;

The auditing implementation classes are all in the package package org.apache.hadoop.fs.s3a.audit.impl. These MUST NOT be subclassed or invoked directly by external code.

Audit classes/interfaces which are intended to be used across object store clients and manipulated from other parts of hadoop are in hadoop-common JAR in the package org.apache.hadoop.fs.store.

Interface org.apache.hadoop.fs.store.audit.AuditSpan

An AuditSpan audits a single Hadoop FileSystem API operation such as open(Path), rename(Path, Path) or listFiles(Path, Boolean).

public interface AuditSpan extends Closeable {

  String getSpanId();

  String getOperationName();

  long getTimestamp();

  AuditSpan activate();

  void deactivate();

  default void close() {

  boolean isValidSpan();

  void set(String key, String value);

Audit Spans are intended for use within FileSystem clients; that is not visible to applications invoking them.

  1. One AuditSpan is created per Hadoop FS API call.
  2. Each span has the name of the operation and optionally source and destination paths.
  3. A span may be activate()d or deactivate()d. Between these two operations a span is active.
  4. Activation is on a per-thread basis. A single span can be active in multiple threads simultaneously; other spans may be active in other threads.
  5. A single filesystem can have only one active span per thread, but different filesystem instances MAY have different active spans.
  6. All store operations performed on a thread are considered within the active span.
  7. Spans do not explicitly terminate; they just stop being invoked; eventually Garbage Collection should dispose of them.
  8. Every AuditSpan has an ID, which must be unique. A UUID and a counter is the base implementation.
  9. The AuditSpan class does extend Closeable; calling close() simply deactivates the span for that thread.
  10. All FS API calls which return objects which go on to perform FS operations (create(), open(), incremental list calls which return RemoteIterator etc) pass the span into the objects which they return.
  11. As a result, any store IO performed by the returned streams and iterators MUST activate the span before that IO and deactivate it afterwards.
  12. There is also the “Unbonded Span” which is the effective span of an FS when there is no active span.
  13. Calling a store within the unbonded span is generally considered an bug. In the S3A codebase this should never happen outside copy/rename operations, and will be logged at warning level in the Logging Auditor.

interface org.apache.hadoop.fs.store.audit.AuditSpanSource

This interface is implemented by sources of audit spans.

public interface AuditSpanSource<T extends AuditSpan> {

  T createSpan(String operation,
      @Nullable String path1,
      @Nullable String path2)
      throws IOException;

All S3 Auditors implement this interface, as does the AuditManagerS3A. (Implementation note: so do S3AFileSystem and WriteOperationHelper)

When a Hadoop FS API call is made of an S3AFileSystem instance, it calls startOperation on its audit manager; this will relay it to the auditor is bound to.

The auditor then creates and returns a span for the specific operation. The AuditManagerS3A will automatically activate the span returned by the auditor (i.e. assign it the thread local variable tracking the active span in each thread).

Memory Leakage through ThreadLocal misuse

The original implementation of the integration with the S3AFileSystem class contained a critical defect, HADOOP-18091 S3A auditing leaks memory through ThreadLocal references.

The original code was written with the assumption that when the ActiveAuditManagerS3A service was garbage collected, references in its ThreadLocal field would be freed. In fact, they are retained until all threads with references are terminated. If any long-lived thread had performed an s3 operation which created a span, a reference back to the audit manager instance was created whose lifetime was that of the thread

In short-lived processes, and long-lived processes where a limited set of S3AFileSystem instances were reused, this had no adverse effect. Indeed, if the filesystem instances were retained in the cache until the process was shut down, there would be strong references to the owning S3AFileSystem instance anyway.

Where it did have problems was when the following conditions were met 1. Process was long-lived. 2. Long-lived threads in the process invoked filesystem operations on s3a:// URLs. 3. Either S3AFileSystem instances were created repeatedly, rather than retrieved from the cache of active instances. 4. Or, after a query for a specific user was completed, FileSystem.closeAllForUGI(UserGroupInformation) was invoked to remove all cached FS instances of that user.

Conditions 1, 2 and 4 are exactly those which long-lived Hive services can encounter.

Auditing was disabled by default until a fix was implemented.

The memory leak has been fixed using a new class org.apache.hadoop.util.WeakReferenceMap to store a map of thread IDs to active spans. When the S3A filesystem is closed, its audit manager service is stopped and all references to spans removed from the map of thread ID to span.

Weak References are used for the map so that span references do not consume memory even if threads are terminated without resetting the span reference of that thread. There is therefore a theoretical risk that if a garbage collection takes place during execution of a spanned operation, the reference will be lost.

This is not considered an issue as all bounded entry points into the S3A filesystem retain a strong reference to their audit span.

All entry points which return an object which can invoke s3 operations (input and output streams, list iterators, etc.) also retain a strong reference to their span, a reference they activate before performing S3 operations. A factory method is provided to demand-generate a new span if, somehow, these conditions are not met. The “unbounded span” is used here. Except in deployments where fs.s3a.audit.reject.out.of.span.operations is true, invoking S3 operations within the unbounded span are permitted. That option is set to true within S3A test suites. Therefore it is unlikely that any operations are invoked in unbounded spans except for the special case of copy operations invoked by the transfer manager threads. Those are already ignored in the logging auditor, whose unbounded span ignores requests which AWSRequestAnalyzer.isRequestNotAlwaysInSpan() indicates may happen outside of a span. This is restricted to bucket location probes possibly performed by the SDK on instantiation, and copy part/complete calls.

  public static boolean
      isRequestNotAlwaysInSpan(final Object request) {
    return request instanceof CopyPartRequest
        || request instanceof CompleteMultipartUploadRequest
        || request instanceof GetBucketLocationRequest;

Class org.apache.hadoop.fs.audit.CommonAuditContext

This is a class in hadoop-common which provides a context to auditing operations across all instrumented filesystems.

It’s Global Context values are a map of string keys and values, which are constant across all threads. This is where global values such as a process UUID and the class executed by ToolRunner are noted.

The CommonAuditContext.currentAuditContext() call returns a thread local CommonAuditContext which is a thread-local map of keys to string values. It also supports a map of evaluated entries. This is a map of type Map&lt;String, Supplier&lt;String>>. supplier methods/lambda expressions set here are dynamically evaluated when auditors retrieve the values. Spans may be used on different thread from that which they were created. Spans MUST always use the values from the currentAuditContext() in the creation thread.

Memory Usage of CommonAuditContext

The CommonAuditContext map has a ThreadLocal field to store global information which is intended to span multiple operations across multiple filesystems, for example the MapReduce or Spark job ID, which is set in the S3A committers.

Applications and Hadoop code MUST NOT attach context entries which directly or indirectly consumes lots of memory, as the life of that memory use will become that of the thread.

Applications and Hadoop code SHOULD remove context entries when no-longer needed.

If memory leakage is suspected here, set the log org.apache.hadoop.fs.audit.CommonAuditContext to TRACE to log the origin of operations which add log entries.

This will produce a log entry whose stack trace will show the caller chain.

2022-01-26 16:10:28,384 TRACE audit.CommonAuditContext (CommonAuditContext.java:put(149)) - Adding context entry t1
java.lang.Exception: t1
    at org.apache.hadoop.fs.audit.CommonAuditContext.put(CommonAuditContext.java:149)
    at org.apache.hadoop.fs.audit.CommonAuditContext.init(CommonAuditContext.java:192)
    at org.apache.hadoop.fs.audit.CommonAuditContext.createInstance(CommonAuditContext.java:210)
    at org.apache.hadoop.fs.audit.CommonAuditContext.lambda$static$0(CommonAuditContext.java:119)
    at java.lang.ThreadLocal$SuppliedThreadLocal.initialValue(ThreadLocal.java:284)
    at java.lang.ThreadLocal.setInitialValue(ThreadLocal.java:180)
    at java.lang.ThreadLocal.get(ThreadLocal.java:170)
    at org.apache.hadoop.fs.audit.CommonAuditContext.currentAuditContext(CommonAuditContext.java:219)
    at org.apache.hadoop.fs.audit.TestCommonAuditContext.<init>(TestCommonAuditContext.java:54)

class NoopAuditor

This auditor creates spans which doesn’t do anything with the events.


This is not the same as disabling auditing, as it still uses the ActiveAuditManagerS3A class which is the source of memory leaks.

Avoid using it except in tests as there is no benefit -simply significant cost.

class LoggingAuditor

The logging auditor logs operations to the console at DEBUG level (to keep the noise down), and attaches the operation details in the HTTP “referer” header.

It can be configured to raise an exception whenever an S3 API call is made from within the unbonded span. This option primarily for development, as it is how we can verify that all calls are audited/identify where this is not possible.

class ActiveAuditManager interface ActiveAuditManager

The class ActiveAuditManager provides all the support needed for S3AFileSystem to support spans, including * Loading and starting the auditor declared in a Hadoop configuration. * Maintaining a per-thread record of the active audit span * Switching spans on AuditSpan.activate() and reverting to the unbonded span in deactivate() and close(). * Providing binding classes to be passed into the AWS SDK so as to invoke audit operations prior to requests being issued. This is essential to guarantee that all AWS S3 operations will be audited.

It’s a YARN composite service which follows the standard lifecycle. The actual auditor is instantiated initialized and started in its service start phase; closed when the Audit Manager is stopped.

public interface AuditManagerS3A extends Service,
    ActiveThreadSpanSource<AuditSpanS3A> {

   * Get the auditor; valid once initialized.
   * @return the auditor.
  OperationAuditor getAuditor();

   * Create the request handler(s) for this audit service.
   * The list returned is mutable; new handlers may be added.
   * @return list of handlers for the SDK.
   * @throws IOException failure.
  List<RequestHandler2> createRequestHandlers() throws IOException;

   * Return a transfer state change callback which
   * fixes the active span context to be that in which
   * the state change listener was created.
   * This can be used to audit the creation of the multipart
   * upload initiation request which the transfer manager
   * makes when a file to be copied is split up.
   * This must be invoked/used within the active span.
   * @return a state change listener.
  TransferStateChangeListener createStateChangeListener();

   * Check for permission to access a path.
   * The path is fully qualified and the status is the
   * status of the path.
   * This is called from the {@code FileSystem.access()} command
   * and is a soft permission check used by Hive.
   * @param path path to check
   * @param status status of the path.
   * @param mode access mode.
   * @return true if access is allowed.
   * @throws IOException failure
  boolean checkAccess(Path path, S3AFileStatus status, FsAction mode)
      throws IOException;

Using Audit Spans within the S3A Connector

  1. All public FS API calls must be marked as @AuditEntryPoint and initiate a span.
  2. All interfaces which provided a subset of the store API to another class (e.g. listing) MUST pick up the current span, store it, and activate/deactivate the span when invoked. This ensures use across threads.
  3. Methods/classes which operate across threads must store the audit span which was active on their creation/invocation, and activate it in all threads which interact with the FS. This should be automatic if callback interfaces do this.
  4. All S3 SDK request objects MUST be created in the request factory. Add new methods if need be.

Implementing a custom OperationAuditor

This extension point is @Unstable

@InterfaceAudience.LimitedPrivate("S3A auditing extensions")
package org.apache.hadoop.fs.s3a.audit;

A custom OperationAuditor auditor is a class which implements the interface org.apache.hadoop.fs.s3a.audit.OperationAuditor. This SHOULD be done by subclassing org.apache.hadoop.fs.s3a.audit.AbstractOperationAuditor.

It is a YARN service and follows the lifecycle: configured in serviceInit(); start any worker threads/perform startup operations in serviceStart() and shutdown in serviceStop().

In use, it will be instantiated in S3AFileSystem.initialize() and shutdown when the FS instance is closed.

It will be instantiated before the AWS S3 Client is built -it may provide a request handler to be part of the handler chain of the S3 request pipeline.

It will be closed in the FileSystem.close() operation, after the S3 Client is itself closed.

Design Decisions/Review questions

Why use https://audit.example.org/ as referrer host?

IETF requires *.example.org to be unresolvable through DNS, so with a well configured DNS there’s never any host to probe.

It guarantees that there will never be real HTTP requests coming in from that host.

And why hadoop/1/ in the referrer path?

Provenance and versioning.

Why no explicit end to an AuditSpan?

While most API calls have a bounded duration, e.g. getFileStatus(), some calls have a very long lifespan (input and output streams). List iterators are never formally terminated, they just “fall out of scope”, Thus, they’d never end.

Having a uniform “Audit Spans are never explicitly terminated” design means that it is consistent everywhere.

Can you activate an already active audit span?

It’s a no-op.

It does mean that if you deactivate the span the first time, then the thread reverts immediately to the unbonded span.

Why does AuditSpan.deactivate() switches to the unbound span, rather than the span which was active before

Again, it gets complicated fast, especially when audit spans our shared across threads.

Because of the sharing you cannot store the previous span in a field within the AuditSpan itself.

Instead you need to have a thread local stack per FileSystem instance of active audit spans.

And you had better be confident that audit spans are correctly activated and deactivated, with no span deactivated more than once -else the stack will become confused.

Having a simple “In Span” or “Out of Span” model avoids this problem. However, it does prevent the S3A FileSystem implementation methods from calling other methods which create new spans. Hence the annotation of all span entry points as @AuditEntryPoint and a need for rigorous review of the invocations. As with the need to make sure that we never call retry() around a method tagged @Retry, making sure that an audit entry point doesn’t invoke another audit entry point is going to become another piece of maintenance overhead.


  • 2021-02 Creation. HADOOP-17511 Add an Audit plugin point for S3A auditing/context.