GUID

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GUID , or Global Unique Identifier, is a set of characters used to identify an object and which is guaranteed to be unique within a system.

Contents 1 Benefits 2 Drawbacks 3 Implementations 3.1 Database Implementation

[edit] Benefits

• Uniqueness of identifiers prevents conflicts. Referencing something by just its identifier is sufficient because nothing else within the system can have the same identifier.
• Comparison of data can often be simplified with GUIDs. To compare any two objects within the system, only the GUIDs need to be compared to determine if they are referring to the exact same thing. The other properties of each object do not need to be compared.
• In larger systems with multiple subsystems, ranges of GUIDs can be used within each subsystem to prevent clashes in the overall system. This is especially useful with multi-master database replication.

[edit] Drawbacks

• Implementation of GUIDs can sometimes be difficult. Typically it's critical they be implemented early in a system design.
• Maintaining uniqueness across multiple systems adds administrative complexity.

[edit] Implementations

There are a variety of ways to implement and use GUIDs.

[edit] Database Implementation

For software using shared databases it's possible to maintain a simple GUID within one database table. This is especially useful with multi-master database replication, where each master must provide unique record identifiers that will not later conflict when the databases are synchronized.

One table will store the current GUID for each master database server. Let's assume we have three master servers. The first will start with GUID 1, the second with 1 million, and the third with 2 million. This gives the first and second servers a set of 1 million GUIDs to work with.

Note this is just an example. A real implementation must thoroughly consider how quickly a range of GUIDs will be used. For some systems with vast amounts of data, the largest integer may not be large enough when shared among multiple databases. This implementation with bigint is typically sufficient for an reasonably large website.

CREATE TABLE guid (
  server VARCHAR(255) NOT NULL,
  id BIGINT UNSIGNED NOT NULL,
  PRIMARY KEY (server));
 
INSERT INTO guid (server, id) VALUES ('db1', 1);
INSERT INTO guid (server, id) VALUES ('db2', 1000000);
INSERT INTO guid (server, id) VALUES ('db3', 2000000);

The software written to use this database system must now always retrieve GUIDs from the guid table by locking it for writes, selecting the current id based on the server name, increasing the id by the number of GUIDs required, and then unlocking the table.

Of course there will come a time when one of the servers runs out of GUIDs. Before that point the GUIDs must be incremented to non-conflicting ranges.

LOCK TABLES guid WRITE;
UPDATE guid SET id = 3000000 WHERE server = 'db1';
UPDATE guid SET id = 4000000 WHERE server = 'db2';
UPDATE guid SET id = 5000000 WHERE server = 'db3';
UNLOCK TABLES;

This effectively gives each server another batch of 1 million GUIDs to work with.