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Chronicle Map - Features

Table of Contents

Chronicle Map is an in-memory, key-value store, designed for low-latency, and/or multi-process applications, such as trading and financial market applications.

Features

  • Ultra low latency: Chronicle Map targets median latency of both read and write queries of less than 1 microsecond in certain tests.

  • High concurrency: Write queries scale well up to the number of hardware execution threads in the server. Read queries never block each other.

  • Persistence to disk - (Optional)

  • Multi-master replication - (Optional, commercial functionality) - Eventually-consistent, fully-redundant, asynchronous replication across servers, "last write wins" strategy by default, allows to implement custom state-based CRDT strategy.

Unique features

  • Multiple processes can access a Chronicle Map concurrently. At the same time, the data store is in-process for each of the accessing processes. Out-of-process approach to IPC is simply incompatible with Chronicle Map’s median latency target of < 1 μs.

  • Replication without logs, with constant footprint cost, guarantees progress even if the network doesn’t sustain write rates.

Note
 See Chronicle Map Replication for more information.

Chronicle Map has two meanings:

From the Java perspective

ChronicleMap is a ConcurrentMap implementation which stores the entries off-heap, serializing/deserializing key and value objects to/from off-heap memory transparently. Chronicle Map supports:

  • Key and value objects caching/reusing for making zero allocations (garbage) on queries.

  • Flyweight values for eliminating serialization/deserialization cost and allowing direct read/write access to off-heap memory.

Primary Chronicle Map use cases

  • Replacing slower key-value stores, like Redis and Memcached, when used within a single server.

  • Replacing similar JVM-centric solutions, like Coherence and Hazelcast, for speed and/or certain Chronicle Map features which those solutions lack.

  • Moving parts of the application-state out of the Java heap for either:

    • Reducing the heap size, for reducing garbage collection pressure, or fitting 32 GB, for using CompressedOops.

    • Inter-process communication.

    • Persistence.

    • Replication across servers.

  • Drop-in ConcurrentHashMap replacement; Chronicle Map performs better in some cases.

What guarantees does Chronicle Map provide in ACID terms?

  • Atomicity - single-key queries are atomic if Chronicle Map is properly configured, multi-key queries are not atomic.

  • Consistency - doesn’t make sense for key-value stores

  • Isolation - yes; for both single- and multi-key queries.

  • Durability - no; Chronicle Map can be persisted to disk, but with no guarantee as to how frequently this happens. This is under the control of the OS. All data is guaranteed to be written to disk when the Map is closed.

  • Clustering and replication for Chronicle Map is provided by Chronicle Map Enterprise.

What is the data structure of Chronicle Map?

Simply put, a Chronicle Map data store is a big chunk of shared memory (optionally mapped to disk).

It is split into independent segments; each segment has:

  • an independent memory allocation for storing the entries

  • a hash table for search

  • a lock in shared memory (implemented via CAS loops) for managing concurrent access.

Chronicle Map is not

  • A document store. There are no secondary indexes.

  • A multimap. Using a Chronicle Map collection as a multimap is technically possible, but often leads to problems. See StackOverflow answer for details. Developing a proper multimap, using Chronicle Map’s design principles is possible.

Please contact us at [email protected] if you would consider sponsoring such development.

Chronicle Map does not support:

  • range queries.

  • iteration over the entries in alphabetical order.

  • sorting of keys.

  • LRU entry eviction.

Features

Table 1. Features
Feature Availability

In-memory off-heap Map

Open source

Persistence to disk

Open source

Remote calls

Commercially available

Eventually-consistent replication (100% redundancy)

Commercially available

Synchronous replication

Commercially available

Partially-redundant replication

On demand

Entry expiration timeouts

On demand
Note
 For access to commercially available features please contact [email protected].

Peer projects

  • Chronicle Datagrid. Reactive processing framework supporting Chronicle Map as a backend.

  • Chronicle Journal. Another key-value built by Chronicle Software, with different properties.