Correct Way to Use Persistent Term

Have been trying to figure out the correct way to code and use a persistent term, and am stuck.

  1. How do I assign a variable to a persistent term that I can access outside the module it was created in?
  2. How would I write the following code?

def build_store do

#Setup Persistent Term variable here

#Stream CSV File. I’m ok with this part using!() and parsing using split
|> x -> #Add to persistent term here# end) #This bit has me stumped


#Once term is created
#How do I read it here from another module?

Any assistance will be greatly appreciated :grinning:

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:persistent_term is VM-wide storage - so accessible from any module. This can create a bit of a challenge for naming so I typically use {module_name, key} as keys for persistent term.

The other consideration is balancing between a persistent storage and an amount of copying. Therefore I find myself using :persistent term when I can flatten the structure somewhat. For example, instead of storing a full map, I store each key of the map (and its value) in :persistent_term.

So to your example: Any module can access :persistent_term so stream and store to your hearts content.


Thx Kip.

Do you perhaps have a code sample of the correct use of persistent term? I have searched high and low and been unsuccessful in finding one.

I don’t seem to have a good example immediately to hand. I do have an example, but arguably its not a good one. Anyway, you’re welcome to whatever value it may contribute:

Really, its a very niche, almost experimental, use case to emit a logger warning only once. Probably not a good example, but the only one I have to hand at the moment.


Thx Kip,

Those examples are actually quite helpful. Please confirm if my understanding is correct. You basically use it as follows:

  1. Create a unique key with the structure: modulename_key e.g mynamestore_firstname, mynamestore_lastname, mypricestore_socks

  2. Save your value as:
    :persistent_term.put(“mynamestore_firstname”, “Mark”)
    :persistent_term.put(“mynamestore_lastname”, “Johnson”)
    :persistent_term.put(“mypricestore_socks”, “25.00”)

  3. Then to access you use:

first_name = :persistent_term.get(“mynamestore_firstname”)
last_name = :persistent_term.get(“mynamestore_lastname”)
sock_price = first_name = :persistent_term.get(“mypricestore_socks”)

Is that correct?

Be sure to check out the performance implications of using persistent_term:

Namely updating and deleting terms may result in a global garbage collection pass that may or may not be acceptable on your use case.


Thx Nicd,

Yes, that is a very important consideration. The use case is a global lookup table that is only read once from a csv file and never updated. I believe it is ok in this use case. Is that correct?


Yes AFAIK that would be a good use case for persistent_term.


I have tried this code and it is not working:

defmodule LookupTable do

def build_lookup do

load_path = Path.join([".", "data", "lookup.csv"])!(load_path,[:read, :utf8]) #Read File
|> #Trim Each Element
|>, [","])) #Split at Delimiters
|> n -> :persistent_term.put(hd(n),tl(n)) end)




I get the following error:

** (UndefinedFunctionError) function :persisent_term.get/1
is undefined (module :persisent_term is not available)

:persistent_term is only available on OTP 21.2 and later so I suspect you are on an older OTP release.


Thx Kip,

I did upgrade both Erlang and Elixir to the latest release. :persistent_term does work in iex.

You have a typo in your code :slight_smile:
You have persisent_term not persistent_term on the last line.


Thx, now I really feel daft :crazy_face:

1 Like

About this

reading the :persistent_term documentation they say

When a persistent term is updated 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.

scan all processes: does it mean all the processes running in the VM, or just the processes that previously were using the delete/updated term?
if I have 100,000 processes and just few of them are dealing with persistent term?

As the docs said, all processes needs to be scanned to see which of them have a reference to the PT. There is no list of processes which have a reference, as the reference from the process might have been collected already via a regular GC, or the process might have sent copies of the reference to another process.

Keeping track of this would add additional complexity.


Cowboy’s documentation is also mentioned about simple usage of persistent_term.

See |> Using persistent_term