public BootstrapSession getBootstrapSession(@Nonnull PartitionId partitionId, @Nonnull String bootstrappingNodeId) {
    Objects.requireNonNull(partitionId, "partitionId cannot be null");
    Objects.requireNonNull(bootstrappingNodeId, "bootstrappingNodeId cannot be null");
    validateBootstrapSessionManagerIsRunningOrThrow();

    return bootstrapSessionsMap.getOrDefault(bootstrappingNodeId, new HashMap<>())
        .getOrDefault(partitionId.getId(), null);
  }
Gets the bootstrap session for a given partition ID and bootstrapping node ID.
@param partitionId The partition ID for the session.
@param bootstrappingNodeId The bootstrapping node ID for the session.
returns the bootstrap session if it exists, null otherwise.
ambry-store/src/main/java/com/github/ambry/store/BootstrapSessionManager.java