> For the complete documentation index, see [llms.txt](https://nodekit.gitbook.io/nodekit-documentation/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nodekit.gitbook.io/nodekit-documentation/architecture/sidecar.md).
# Sidecar
Chains leverage Javelin by running a sidecar. The sidecar runs alongside the chains validators/sequencers and allows them to opt into a shared auction that runs across all the chains in the network to help facilitate cross-chain composability.
Chains register ahead of time to this shared auction and this allows Javelin builders to bid on a shared slot across chains. These shared slots are done in epochs that last 12 seconds. A builder bids for the right to build across all chains in the Composium for a given epoch.
Blocks are built and sent to the chain in time with the chain’s blocktime. Builders switch every epoch. Chains benefit by unlocking cross-chain composability with Javelin while being able to maintain flexibility in their stack and allowing them to have their own block time without giving up any revenue.
Chains benefit by unlocking cross-chain composability with Javelin while being able to maintain flexibility in their stack and allowing them to have their own block time without giving up any revenue.
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