If you are looking how to upgrade your router or migrate from one router compatibility version to another, please see Upgrading the Router.

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What is a router compatibility version?

In short, a new router compatibility version designates a breaking change or breaking behaviour within the contract between composition, the router execution configuration, and the router version. When composing a federated graph, Cosmo relies on its composition library (within the same mono-repository), to perform the following:
  1. Validate that all the subgraphs composing that federated graph are compatible and produce a fully-functioning federated graph.
  2. Produce a router execution configuration.
Each router execution configuration that is produced also defines the compatibilityVersion property. Older router execution configurations will not include this property, but those configuration are still simply considered to be version 1. The compatibilityVersion property itself comprises two distinct items, delimited by a colon (:).
  1. The router compatibility version.
  2. The composition library package version.
For instance, the following (reduced) router execution configuration JSON defines router compatibility version 1 and composition library package version 0.1.0. 
{
 ...
 "compatibilityVersion": "1:0.1.0"
}
Each version of the router internally defines a threshold for the router compatibility version with which it can function. If a router execution configuration with a higher router compatibility version than the the threshold of the router is provided, the router will produce an error and will not start. This relationship between a router compatibility version defined in a router execution configuration and the router compatibility version threshold defined in a router can be considered as a handshake between a router and its execution configuration. Proper functionality and behaviour of the router can only be guaranteed when this handshake is successful.

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Why are router compatibility versions necessary?

New router compatibility versions are not intended to be regular occurrences. They will only be released when absolutely necessary—i.e., when backwards compatibility would be unfeasible. A typical circumstance would be when the router execution configuration requires a structural change; or the data provided would affect how older router versions would handle and execute that data.

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How does Cosmo Cloud store different execution configurations with different router compatibility versions?

In short, router execution configurations are discretely stored by their router compatibility version. A router will only retrieve and poll the execution configuration stored at the address corresponding with its router compatibility version threshold. For instance, before upgrading your valid federated graph to router compatibility version 2, it will have been successfully composed under version 1. The Cosmo CDN stores the latest router execution configuration created under router compatibility version 1 (or earlier configurations with no versions) at one address, and “versioned” configurations at different bespoke addresses. The router compatibility version threshold of a router dictates from where that router will receive and poll its execution configuration. So, a router with a threshold of <=1will receive and poll its execution config from the original storage address, and a router with a threshold of 2 will receive and poll its execution config from the corresponding version address. Please see wgc federated-graph version set for full details.

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Which router compatibility versions are currently supported?

Please see the router compatibility-version list command.