# Re \`seed!\` ing the RNG at each step of a simulation?
**URL:** https://discourse.julialang.org/t/re-seed-ing-the-rng-at-each-step-of-a-simulation/132158
**Category:** General Usage
**Tags:** question, random
**Created:** [September 7, 2025, 5:40pm UTC](https://discourse.julialang.org/t/re-seed-ing-the-rng-at-each-step-of-a-simulation/132158 "2025-09-07T17:40:05Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@nhz2](https://discourse.julialang.org/u/nhz2)
#### Post date: [September 7, 2025, 5:40pm UTC](https://discourse.julialang.org/t/re-seed-ing-the-rng-at-each-step-of-a-simulation/132158/1 "2025-09-07T17:40:05Z")
I am running long stochastic simulations that look like:
```julia
sim_name = "example/1"
Random.seed!(sim_name)
ctx = setup(sim_name)
for frame in 1:1000
ctx = loop(ctx)
end
```
I need to be able to restart simulations that crash (for example, due to hardware issues). I have defined serialization functions for `ctx`, but from what I can tell, there is no stable way to serialize the state of the default RNG because its implementation can change between minor Julia versions.
Is reseeding the default RNG on each step a valid alternative?
```julia
sim_name = "example/1"
Random.seed!("$(sim_name)/$(0)")
ctx = setup(sim_name)
for frame in 1:1000
Random.seed!("$(sim_name)/$(frame)")
ctx = loop(ctx)
end
```
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [September 7, 2025, 7:10pm UTC](https://discourse.julialang.org/t/re-seed-ing-the-rng-at-each-step-of-a-simulation/132158/2 "2025-09-07T19:10:07Z")
> [@nhz2](#):
>
> Is reseeding the default RNG on each step a valid alternative?
No, since this is also not reproducible across Julia versions (and may be less random).
The best approach depends a bit on what you are doing. How is randomness used in your simulation?
e.g. you could use StableRNGs.jl if you restrict yourself to random sampling functions in its stable API.
---
### Author:  [@nhz2](https://discourse.julialang.org/u/nhz2)
#### Post date: [September 7, 2025, 7:21pm UTC](https://discourse.julialang.org/t/re-seed-ing-the-rng-at-each-step-of-a-simulation/132158/3 "2025-09-07T19:21:27Z")
I’m okay with it only being reproducible in a single Julia version, but what do you mean by it being less random?
I am simulating a jump process; the RNG is used to randomly select the next event, and when that event happens, the event callback then also uses the RNG to determine what happened or reject the event.
I could store the RNG in `ctx` and then rewrite all the event callbacks to use the RNG in `ctx` instead of the default one.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [September 7, 2025, 8:54pm UTC](https://discourse.julialang.org/t/re-seed-ing-the-rng-at-each-step-of-a-simulation/132158/4 "2025-09-07T20:54:12Z")
> [@nhz2](#):
>
> I’m okay with it only being reproducible in a single Julia version
Then what’s the problem with serializing the RNG state?
> [@nhz2](#):
>
> but what do you mean by it being less random?
If you re-seed too often it changes the statistics. (Imagine the extreme case where you re-seed after each random number.)
---
### Author:  [@nhz2](https://discourse.julialang.org/u/nhz2)
#### Post date: [September 7, 2025, 9:43pm UTC](https://discourse.julialang.org/t/re-seed-ing-the-rng-at-each-step-of-a-simulation/132158/5 "2025-09-07T21:43:26Z")
Yes, that works:
```julia-repl
julia> using Base64, Serialization, Random
julia> function rng2str()
base64encode(sprint(serialize, copy(Random.default_rng())))
end
rng2str (generic function with 1 method)
julia> function str2rng(s)
copy!(Random.default_rng(), deserialize(IOBuffer(base64decode(s))))
nothing
end
str2rng (generic function with 1 method)
julia> s = rng2str()
"N0pMGgQAAAA1EAEHWG9zaGlybx8LXP2hgFmEEZoCX8mihII/mgEGUmFuZG9tRAlpUUqIGas5+QnpupKUwWTyqAms62WLbMY8vAngTB2njJ+GtwmHsEKhRY8d/g=="
julia> rand()
0.29811661892484653
julia> str2rng(s)
julia> rand()
0.29811661892484653
```
Though using `deserialize` doesn’t seem ideal because its docstring says “Malformed data can result in process termination.”