persistent_term(3)



persistent_term(3erl)      Erlang Module Definition      persistent_term(3erl)

NAME
       persistent_term - Persistent terms.

DESCRIPTION
       This  module is similar to ets in that it provides a storage for Erlang
       terms that can be accessed in constant time, but  with  the  difference
       that persistent_term has been highly optimized for reading terms at the
       expense of writing and updating terms. When a persistent  term  is  up-
       dated  or  deleted, a global garbage collection pass is run to scan all
       processes for the deleted term, and to copy it into each  process  that
       still  uses  it. Therefore, persistent_term is suitable for storing Er-
       lang terms that are frequently accessed but never or  infrequently  up-
       dated.

   Warning:
       Persistent  terms  is an advanced feature and is not a general replace-
       ment for ETS tables. Before using persistent terms, make sure to  fully
       understand  the  consequence  to  system  performance  when updating or
       deleting persistent terms.

       Term lookup (using get/1), is done in constant time and without  taking
       any locks, and the term is not  copied to the heap (as is the case with
       terms stored in ETS tables).

       Storing or updating a term (using put/2) is proportional to the  number
       of  already created persistent terms because the hash table holding the
       keys will be copied. In addition, the term itself will be copied.

       When a (complex) term is deleted (using erase/1) or replaced by another
       (using  put/2), a global garbage collection is initiated. It works like
       this:

         * All processes in the system will be scheduled  to  run  a  scan  of
           their  heaps for the term that has been deleted. While such scan is
           relatively light-weight, if there are many  processes,  the  system
           can  become  less  responsive  until all process have scanned their
           heaps.

         * If the deleted term (or any part of it) is still used by a process,
           that  process  will  do  a major (fullsweep) garbage collection and
           copy the term into the process. However, at most two processes at a
           time will be scheduled to do that kind of garbage collection.

       Deletion  of atoms and other terms that fit in one machine word is spe-
       cially optimized to avoid doing a global GC. It  is  still  not  recom-
       mended  to  update persistent terms with such values too frequently be-
       cause the hash table holding the keys is copied every time a persistent
       term is updated.

       Some examples are suitable uses for persistent terms are:

         * Storing of configuration data that must be easily accessible by all
           processes.

         * Storing of references for NIF resources.

         * Storing of references for efficient counters.

         * Storing an atom to indicate a logging level or whether debugging is
           turned on.

STORING HUGE PERSISTENT TERMS
       The current implementation of persistent terms uses the literal alloca-
       tor also used for literals (constant terms) in BEAM code. By default, 1
       GB  of  virtual address space is reserved for literals in BEAM code and
       persistent terms. The amount of virtual address space reserved for lit-
       erals  can be changed by using the +MIscs option when starting the emu-
       lator.

       Here is an example how the reserved virtual address space for  literals
       can be raised to 2 GB (2048 MB):

           erl +MIscs 2048

BEST PRACTICES FOR USING PERSISTENT TERMS
       It is recommended to use keys like ?MODULE or {?MODULE,SubKey} to avoid
       name collisions.

       Prefer creating a few large persistent terms  to  creating  many  small
       persistent  terms.  The execution time for storing a persistent term is
       proportional to the number of already existing terms.

       Updating a persistent term with the same value as  it  already  has  is
       specially  optimized to do nothing quickly; thus, there is no need com-
       pare the old and new values and avoid calling put/2 if the  values  are
       equal.

       When  atoms or other terms that fit in one machine word are deleted, no
       global GC is needed. Therefore, persistent terms  that  have  atoms  as
       their  values  can  be  updated more frequently, but note that updating
       such persistent terms is still much more expensive than reading them.

       Updating or deleting a persistent term will trigger a global GC if  the
       term  does  not fit in one machine word. Processes will be scheduled as
       usual, but all processes will be made runnable at once, which will make
       the system less responsive until all process have run and scanned their
       heaps for the deleted terms. One way to minimize the effects on respon-
       siveness  could  be to minimize the number of processes on the node be-
       fore updating or deleting a persistent term. It would also be  wise  to
       avoid updating terms when the system is at peak load.

       Avoid  storing a retrieved persistent term in a process if that persis-
       tent term could be deleted or updated in the future. If a process holds
       a  reference to a persistent term when the term is deleted, the process
       will be garbage collected and the term copied to process.

       Avoid updating or deleting more than one persistent  term  at  a  time.
       Each  deleted  term  will  trigger  its  own global GC. That means that
       deleting N terms will make the system less responsive  N  times  longer
       than deleting a single persistent term. Therefore, terms that are to be
       updated at the same time should be collected into a  larger  term,  for
       example, a map or a tuple.

EXAMPLE
       The  following  example shows how lock contention for ETS tables can be
       minimized by having one ETS table for each scheduler. The table identi-
       fiers for the ETS tables are stored as a single persistent term:

           %% There is one ETS table for each scheduler.
           Sid = erlang:system_info(scheduler_id),
           Tid = element(Sid, persistent_term:get(?MODULE)),
           ets:update_counter(Tid, Key, 1).

DATA TYPES
       key() = term()

              Any Erlang term.

       value() = term()

              Any Erlang term.

EXPORTS
       erase(Key) -> Result

              Types:

                 Key = key()
                 Result = boolean()

              Erase  the name for the persistent term with key Key. The return
              value will be true if there was a persistent term with  the  key
              Key,  and  false if there was no persistent term associated with
              the key.

              If there existed a previous persistent term associated with  key
              Key,  a  global  GC has been initiated when erase/1 returns. See
              Description.

       get() -> List

              Types:

                 List = [{key(), value()}]

              Retrieve the keys and values for all persistent terms. The  keys
              will  be  copied  to the heap for the process calling get/0, but
              the values will not.

       get(Key) -> Value

              Types:

                 Key = key()
                 Value = value()

              Retrieve the value for the persistent term associated  with  the
              key  Key. The lookup will be made in constant time and the value
              will not be copied to the heap of the calling process.

              This function fails with a badarg exception if no term has  been
              stored with the key Key.

              If  the  calling process holds on to the value of the persistent
              term and the persistent term is deleted in the future, the  term
              will be copied to the process.

       get(Key, Default) -> Value

              Types:

                 Key = key()
                 Default = Value = value()

              Retrieve  the  value for the persistent term associated with the
              key Key. The lookup will be made in constant time and the  value
              will not be copied to the heap of the calling process.

              This  function  returns  Default if no term has been stored with
              the key Key.

              If the calling process holds on to the value of  the  persistent
              term  and the persistent term is deleted in the future, the term
              will be copied to the process.

       info() -> Info

              Types:

                 Info = #{count := Count, memory := Memory}
                 Count = Memory = integer() >= 0

              Return information about persistent terms in a map. The map  has
              the following keys:

                count:
                  The number of persistent terms.

                memory:
                  The  total  amount of memory (measured in bytes) used by all
                  persistent terms.

       put(Key, Value) -> ok

              Types:

                 Key = key()
                 Value = value()

              Store the value Value as a persistent term and associate it with
              the key Key.

              If  the  value Value is equal to the value previously stored for
              the key, put/2 will do nothing and return quickly.

              If there existed a previous persistent term associated with  key
              Key,  a global GC has been initiated when put/2 returns. See De-
              scription.

Ericsson AB                       erts 11.0.2            persistent_term(3erl)

Man(1) output converted with man2html
list of all man pages