Interface PersistentDataStore

  • All Superinterfaces:

    public interface PersistentDataStore
    Interface for a data store that holds feature flags and related data in a serialized form.

    This interface should be used for database integrations, or any other data store implementation that stores data in some external service. The SDK will take care of converting between its own internal data model and a serialized string form; the data store interacts only with the serialized form. The SDK will also provide its own caching layer on top of the persistent data store; the data store implementation should not provide caching, but simply do every query or update that the SDK tells it to do.

    Implementations must be thread-safe.

    Conceptually, each item in the store is a DataStoreTypes.SerializedItemDescriptor which always has a version number, and can represent either a serialized object or a placeholder (tombstone) for a deleted item. There are two approaches a persistent store implementation can use for persisting this data:

    1. Preferably, it should store the version number and the DataStoreTypes.SerializedItemDescriptor.isDeleted() state separately so that the object does not need to be fully deserialized to read them. In this case, deleted item placeholders can ignore the value of DataStoreTypes.SerializedItemDescriptor.getSerializedItem() on writes and can set it to null on reads. The store should never call DataStoreTypes.DataKind.deserialize(String) or DataStoreTypes.DataKind.serialize(DataStoreTypes.ItemDescriptor).

    2. If that isn't possible, then the store should simply persist the exact string from DataStoreTypes.SerializedItemDescriptor.getSerializedItem() on writes, and return the persisted string on reads (returning zero for the version and false for DataStoreTypes.SerializedItemDescriptor.isDeleted()). The string is guaranteed to provide the SDK with enough information to infer the version and the deleted state. On updates, the store must call DataStoreTypes.DataKind.deserialize(String) in order to inspect the version number of the existing item if any.

    Error handling is defined as follows: if any data store operation encounters a database error, or is otherwise unable to complete its task, it should throw a RuntimeException to make the SDK aware of this. The SDK will log the exception and will assume that the data store is now in a non-operational state; the SDK will then start polling isStoreAvailable() to determine when the store has started working again.

    • Method Detail

      • init

        void init​(DataStoreTypes.FullDataSet<DataStoreTypes.SerializedItemDescriptor> allData)
        Overwrites the store's contents with a set of items for each collection.

        All previous data should be discarded, regardless of versioning.

        The update should be done atomically. If it cannot be done atomically, then the store must first add or update each item in the same order that they are given in the input data, and then delete any previously stored items that were not in the input data.

        allData - a list of DataStoreTypes.DataKind instances and their corresponding data sets
      • upsert

        boolean upsert​(DataStoreTypes.DataKind kind,
                       java.lang.String key,
                       DataStoreTypes.SerializedItemDescriptor item)
        Updates or inserts an item in the specified collection.

        If the given key already exists in that collection, the store must check the version number of the existing item (even if it is a deleted item placeholder); if that version is greater than or equal to the version of the new item, the update fails and the method returns false. If the store is not able to determine the version number of an existing item without fully deserializing the existing item, then it is allowed to call DataStoreTypes.DataKind.deserialize(String) for that purpose.

        If the item's DataStoreTypes.SerializedItemDescriptor.isDeleted() method returns true, this is a deleted item placeholder. The store must persist this, rather than simply removing the key from the store. The SDK will provide a string in DataStoreTypes.SerializedItemDescriptor.getSerializedItem() which the store can persist for this purpose; or, if the store is capable of persisting the version number and deleted state without storing anything else, it should do so.

        kind - specifies which collection to use
        key - the unique key for the item within that collection
        item - the item to insert or update
        true if the item was updated; false if it was not updated because the store contains an equal or greater version
      • isStoreAvailable

        boolean isStoreAvailable()
        Tests whether the data store seems to be functioning normally.

        This should not be a detailed test of different kinds of operations, but just the smallest possible operation to determine whether (for instance) we can reach the database.

        Whenever one of the store's other methods throws an exception, the SDK will assume that it may have become unavailable (e.g. the database connection was lost). The SDK will then call isStoreAvailable() at intervals until it returns true.

        true if the underlying data store is reachable