Hash value

As explained in Akka in Action, Second Edition

A hash code is a deterministic numeric value computed from input data for use in partitioning or lookup.

A hash value is a deterministic numeric result computed from a message key. In consistent hashing, Akka uses it to select a virtual node and then the routee responsible for that node. section 6.3.4

Consistent-hashing routing proceeds by:

• Mapping each message to a stable key, such as its message ID. section 6.3.4
• Computing a hash value from that key. section 6.3.4
• Using the result to choose a virtual node and its routee. section 6.3.4

Stable message routing

If two Aggregator messages have the same ID, the mapping function produces the same key and hash value, so both messages are assigned to the same routee. Virtual nodes also support more even distribution and route changes. section 6.3.4

Figure 6.7 Steps the consistent hashing router follows to decide which routee to select

Follow the routing steps from message key through hash value, virtual node, and selected routee. section 6.3.4

How Akka in Action, Second Edition covers it

How deep it goes

Explains key extraction, hashing, virtual-node selection, and routee selection while noting these internals remain hidden from normal users.

Examples

Uses message IDs, Aggregator messages, data enrichment, and data obfuscation to illustrate stable routing.

What it assumes

Assumes a stable message-key mapping and a set of routees participating in a router.

What you can do after

Apply it via Akka consistent-hashing routers to keep equal-key messages on one routee.

How it connects

Connects message keys, consistent hashing, virtual nodes, routees, and group routers.

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Related concepts

• hash functionproduced by · A hash function deterministically maps input data to a value in a bounded output range, commonly for indexing, lookup, comparison, or integrity checking.
• Mapused by · A map is an associative data structure that stores key-value pairs and supports retrieving values by key.
• hashsetused by · A hash set is a collection of distinct values that uses hashing to support efficient membership testing without associated values.
• cryptographic hashproduced by · A cryptographic hash is a fixed-length digest computed from input data by a deterministic function designed to resist preimage, second-preimage, and collision attacks.
• checksumincludes · A checksum is a value computed from data and used to detect unintended changes or corruption by comparison with a reference value or recomputation.
• password hashingused by · Password hashing is the use of a one-way, deliberately costly function to derive values for securely storing and verifying passwords without retaining the original passwords.

This explainer was generated by Manning's knowledge graph from the text of Akka in Action, Second Edition. The section links point to the parts of the book it draws on.