Oban Met generating JSON encoding error: (Protocol.UndefinedError) protocol JSON.Encoder not implemented

nathanl

I see this error locally when running my application. mix deps shows versions:

oban 2.19.1
oban_met 0.1.11
oban_pro 1.5.0
oban_web 2.10.2
telemetry 1.3.0
telemetry_metrics 1.1.0
telemetry_poller 1.1.0

[error] GenServer {Oban.Registry, {Oban, Oban.Met.Listener}} terminating
** (Protocol.UndefinedError) protocol JSON.Encoder not implemented for type Oban.Met.Values.Gauge (a struct), the protocol must be explicitly implemented.

If you have a struct, you can derive the implementation specifying which fields should be encoded to JSON:

    @derive {JSON.Encoder, only: [....]}
    defstruct ...

It is also possible to encode all fields, although this should be used carefully to avoid accidentally leaking private information when new fields are added:

    @derive JSON.Encoder
    defstruct ...

Finally, if you don't own the struct you want to encode to JSON, you may use Protocol.derive/3 placed outside of any module:

    Protocol.derive(JSON.Encoder, NameOfTheStruct, only: [...])
    Protocol.derive(JSON.Encoder, NameOfTheStruct). This protocol is implemented for the following type(s): Atom, BitString, Date, DateTime, Decimal, Duration, Float, Integer, List, Map, NaiveDateTime, Oban.Pro.Producer, Oban.Pro.Producer.Meta.GlobalLimit, Oban.Pro.Producer.Meta.GlobalLimit.Partition, Oban.Pro.Producer.Meta.RateLimit, Oban.Pro.Producer.Meta.RateLimit.Partition, Time

Got value:

    %Oban.Met.Values.Gauge{data: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]}

    (elixir 1.18.2) lib/json.ex:1: JSON.Encoder.impl_for!/1
    (elixir 1.18.2) lib/json.ex:109: JSON.Encoder.encode/2
    (elixir 1.18.2) src/elixir_json.erl:323: :elixir_json."-do_encode_map/2-lc$^0/1-0-"/2
    (elixir 1.18.2) src/elixir_json.erl:323: :elixir_json.do_encode_map/2
    (elixir 1.18.2) src/elixir_json.erl:308: :elixir_json.do_encode_list/2
    (elixir 1.18.2) src/elixir_json.erl:323: :elixir_json."-do_encode_map/2-lc$^0/1-0-"/2
    (elixir 1.18.2) src/elixir_json.erl:323: :elixir_json."-do_encode_map/2-lc$^0/1-0-"/2
    (elixir 1.18.2) src/elixir_json.erl:323: :elixir_json.do_encode_map/2
    (oban 2.19.1) lib/oban/notifier.ex:307: Oban.Notifier.encode/1
    (elixir 1.18.2) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (oban 2.19.1) lib/oban/notifier.ex:202: anonymous fn/4 in Oban.Notifier.notify/3
    (telemetry 1.3.0) /Users/me/code/myapp/deps/telemetry/src/telemetry.erl:324: :telemetry.span/3
    (oban_met 0.1.11) lib/oban/met/listener.ex:82: Oban.Met.Listener.handle_info/2
    (stdlib 6.2) gen_server.erl:2345: :gen_server.try_handle_info/3
    (stdlib 6.2) gen_server.erl:2433: :gen_server.handle_msg/6
    (stdlib 6.2) proc_lib.erl:329: :proc_lib.init_p_do_apply/3
Last message: :report

7 comments

/oban #bug

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2025-02-25 19:06:36 UTC

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sorentwo

Oban Core Team

Oban added a transparent switch between Jason and JSON, which must also be respected by the other packages. In this case, oban_met is behind a version.

Upgrading oban_web (Changelog — Oban Web v2.12.5) and oban_met (Changelog — Oban Met v1.0.1) to the latest versions will fix the issue:

{:oban_web, "~> 2.11"},
{:oban_met, "~> 1.0"}

Post #1

nathanl

Understood. The newer oban_web requires phoenix_live_view 1.0, which is a bigger lift to upgrade to. (We even have a couple of deps that require an older version of LV.) I guess we’ll have to prioritize that or else use an older version of Oban until we can.

Post #2

sorentwo

Oban Core Team

There are minimal changes in Web v2.11 to support LV 1.0. The newer oban_web may work with a slightly older version of phoenix_live_view if you want to try an override: true.

Post #3

nathanl

Thanks, that will let us limp along for a bit. Specifying the oban versions you showed and {:phoenix_live_view, "~> 0.20.17", override: true} fixes the decoding errors. It means that the Oban Web dashboard just displays {@inner_content} , but we can work to get on LiveView 1.0 before the next deploy.

Post #4

thiagogsr

It seems I cannot use Oban 2.19 on Elixir 1.18 with oban_web 2.10.2 and oban_met 0.1.11 because oban_met only encodes to Jason. I can’t update oban_web yet because we are on phoenix_live_view 0.20.17 it doesn’t work.

Is there a workaround for this? I had to uninstall oban_web temporarily until I find a solution for this without Oban downgrade.

Have you considered a config option on Oban to select the json library?

Post #5

sorentwo

Oban Core Team

Is there a workaround for this?

You can use oban_met v1.0 with override: true to force it. There aren’t any API changes between v0.1.11 and v1.0.0 and oban_web can use it just fine.

Post #6