Recover the State After a Crash

Learn how to hang on to a specific state and restore it if a process happens to exit abnormally.

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Fault tolerance

This is where we fulfill the promise of fault tolerance. It’s one thing to restart a process if it crashes and then move on. It’s another thing entirely to restart it and restore the last known good state.

The way we do this is to save a copy of the data outside the current process, or any other process the current one is linked to. We do this when we initialize the process, and then again whenever the state changes.

Whenever we start a new process or restart a crashed process, we check for that saved state. If it exists, this means we’re restarting, so we’ll use the saved version. If it doesn’t exist, that means it’s a new process, so we’ll use a fresh state.

ETS tables

ETS is the storage engine we use. ETS is short for Erlang Term Storage. ETS comes with OTP, and ETS allows us to store data in in-memory tables as two-element tuples. The first element of each tuple is the key. The value is the second element.

ETS tables offer a number of different options to choose from. These options specify how they store data and which processes can access it.

Types of ETS

There are four different types of ETS tables:

  1. :set is the default type, and :set tables store exactly one value per key.
  2. :ordered_set tables behave the same as :set tables, except that they order the keys.
  3. :bag tables store multiple values under the same key, as long as the values are not exact duplicates.
  4. :duplicate_bag tables store multiple values under the same key, even if they are exact duplicates.

Levels of privacy

There are three levels of privacy:

  1. :private means that only the process that starts the table can read to or write from it.
  2. :protected is the default. Using :protected means the process that starts the table can read from and write to the table. All other processes can only be read from it.
  3. :public means that all processes can read from and write to the table.

There is also :named_table option. This option allows us to reference that table by name instead of needing to keep track of itsreference.

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