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Erlang / Erlang Advanced Interview questions

Why do distributed Erlang nodes require a shared cookie?

The cookie is a shared secret string that every node in a cluster must present to authenticate itself before another node will accept a connection from it — without a matching cookie, a node can't join the cluster, send messages to remote PIDs, or make remote calls.

%% set via command line or ~/.erlang.cookie
erl -sname nodea -setcookie mysecret

Without this check, any process capable of reaching a node's distribution port over the network could attach as a full cluster peer and execute arbitrary code, since distributed Erlang gives connected nodes deep capabilities (spawning processes, calling functions remotely). The cookie is a coarse, cluster-wide gate, not a fine-grained per-user authorization system, which is exactly why distributed Erlang is generally recommended only on trusted, isolated networks rather than exposed directly to the public internet.

What does the shared cookie protect against in distributed Erlang?
Why is the cookie described as a coarse security mechanism?

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What is the difference between linking and monitoring a process? Why would you choose monitor over link for a client process? What is a dirty scheduler and when should a NIF use one? Explain the internal working of Erlang's per-process garbage collector? Why does Erlang use generational (per-process) garbage collection instead of a single global GC? What is EPMD and what role does it play in distributed Erlang? Why do distributed Erlang nodes require a shared cookie? Explain the internal working of the Erlang distribution handshake between two nodes? What is the two-version code loading rule and what happens when a third version is loaded? Explain the internal working of code:purge/1 and code:soft_purge/1? What is a gen_event and when would you choose it over gen_server? What is the difference between error, exit, and throw in Erlang? Why is exit/1 different from exit/2? What is an OTP application (.app file) and how does it differ from a single module? Explain the execution flow of application:start/1 and its dependency resolution? What is a release in OTP and how does it differ from an application? Explain the internal working of a relup-based hot upgrade? What is the code_change/3 callback used for in gen_server? How does the global module handle process name registration across a cluster? Why can global name registration cause a network partition ("split brain") problem? What is the pg (process groups) module used for? What are Erlang maps and how do they differ from records? When should you choose a map over a record for structured data? What is the difference between ets:match, ets:select, and ets:foldl? How can you use match specifications to filter ETS data efficiently? What do the write_concurrency and read_concurrency ETS options optimize for? How can you optimize binary pattern matching for parsing variable-length network protocols? Why is copying a large list between processes more expensive than copying a large binary? What is tail call optimization in Erlang and why does it matter for long-running loops? How do you write a properly tail-recursive accumulator-based function? Explain the internal working of selective receive and why message order in the queue matters? What is erlang:process_flag(priority,...) used for? Why should high-priority processes be used sparingly in Erlang? What is rpc:call/4 and how does it work across distributed nodes? What is the difference between rpc:call and simply sending a message to a remote PID? How does erlang:send/3 with the nosuspend option change message-sending behavior? What is Dialyzer and how does success typing differ from static typing? Why doesn't Dialyzer catch every type error the way a traditional type checker would? How do you define and use a custom behavior with the -callback attribute? What is the difference between a behavior callback module and a plain library module? Explain the internal working of exception propagation through nested try/catch blocks? When should you use throw instead of returning an {error, Reason} tuple? How do you troubleshoot a memory leak caused by large binaries not being garbage collected? What is the significance of the binary reference count and off-heap binary garbage collection? Why is the Erlang distribution protocol not encrypted by default, and how can you secure inter-node traffic? Why is Mnesia's two-phase commit necessary for distributed transactions? How do you troubleshoot slow ETS lookups on a heavily-used table? Why might increasing the number of BEAM schedulers not improve throughput linearly?
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