Prelude
Some notes from the Envoy Internals Deep Dive talk by Matt Klein. The talk was given at KubeCon EU 2018. Watch on YouTube here.
Actual Notes :D
Architecture
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Envoy is designed with an out of process architecture specifically to address the polyglot nature of microservices
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At its core, Envoy is a byte proxy. This gives it extensibility to supoort a multitude of protocols.
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Create with versatility from the start so that Envoy can act as service, middle, and edge proxy
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Hot restart is key feature
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One part of Envoy is built as a pipeline with filters as building blocks. Filters are extension points where users can add functionality.
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The other part of the architecture is where the central management functionality lives. Things like the cluster/service managers or stats engine.
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The prevailing school of thought for proxies before the 2000s was to have serve a single connection per thread. Envoy is takes a different approach where it serves multiple connections per thread via an event loop.
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The “main” thread of Envoy handles the management functionality and worker threads handle the connections and filter pipeline. The key to keeping Envoy simple is the latter part of the previous statement. It means that worker threads do not have to communicate with each other and only have to acquire locks in a small set of cases.
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Read-Copy-Update (RCU) is used to govern parallelization within Envoy. “Designed for read heavy, write infrequent workloads”. Helps reduce the need for locking by making the read path lock free.
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Envoy keeps a vector pointers in thread local storage (TLS) that is mirrored across the main and worker threads. RCU is used to update slots in the vector and posts messages to worker threads to update information e.g. route tables.
Hot Restarts
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“Full binary reload without dropping any connections”
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Allow two processes of Envoy to run at the same time and utilize a shared memory region. Stats and locks are stored here.
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The two proceses communicate through an RPC protocol over Unix domain sockets (UDS) and perform socket passing. After the second Envoy process spins up, it will communicate that it is good to go and the first process will begin to drain connections.
Stats
- There’s a 2 level caching mechanism for stats. There is a TLS cache, a central cache on the main thread, and the actual entries either in shared memory or in process memory.
Useful references
- Link to talk on YouTube
- Slides from the talk
- Link to Matt Klein’s Medium - ALL HIS ARTICLES ARE HIGHLY
RECOMMENDED READING
- Especially this one on modern load balancing