Is there a limit on the size of a string in JSON with Node.js?

The maximum length of a string in node.js is defined by the underlying Javascript Engine "V8". In V8 the maximum length is independent of the heap size. The size of a string is actually constrained by the limits defined by optimized object layout. See https://chromium-review.googlesource.com/c/v8/v8/+/2030916 which is a recent (Feb 2020) change to the maximum length of a string in V8. The commit message explains the different lengths over time. The limit has gone from about 256MB to 1GB then back to 512MB (on 64-bit V8 platforms).

V8 (the JavaScript engine node is built upon) until very recently had a hard limit on heap size of about 1.9 GB.

Node v0.10 is stuck on an older version of V8 (3.14) due to breaking V8 API changes around native addons. Node 0.12 will update to the newest V8 (3.26), which will break many native modules, but opens the door for the 1.9 GB heap limit to be raised.

So as it stands, a single node process can keep no more than 1.9 GB of JavaScript code, objects, strings, etc combined. That means the maximum length of a string is under 1.9 GB.

You can get around this by using Buffers, which store data outside of the V8 heap (but still in your process's heap). A 64-bit build of node can pretty much fill all your RAM as long as you never have more than 1.9 GB of data in JavaScript variables.

All that said, you should never come anywhere near this limit. When dealing with this much data, you must deal with it as a stream. You should never have more than a few megabytes (at most) in memory at one time. The good news is node is especially well-suited to dealing with streaming data.

You should ask yourself some questions:

What kind of data are you actually receiving from the user?
Why do you want to store it in JSON format?
Is it really a good idea to stuff gigabytes into JSON? (The answer is no.)
What will happen with the data later, after it is stored? Will your code read it? Something else?

The question you've posted is actually quite vague in regard to what you're actually trying to accomplish. For more specific advice, update your question with more information.

If you expect the data to never be all that big, just throw a reasonable limit of 10 MB or something on the input, buffer it all, and use JSON.stringify.

If you expect to deal with data any larger, you need to stream the input straight to disk. Look in to transform streams if you need to process/modify the data before it goes to disk. For example, there are modules that deal with streaming JSON.

This is a good question, but I think the upper limit you need to be worried about doesn't involve the max JSON string size.

In my opinion the limit you need to worry about is how long do you wish to block the request thread while it's processing the user's request.

Any string size over 1MB will take the user a few seconds to upload and 10 of Megabytes could take minutes. After receiving the request, the server will take a few hundred milliseconds to seconds to parse into a data structure leading to a very poor user experience (Parsing JSON is very expensive)

The bandwidth and server processing times will overshadow any limit JSON may have on string size.

The maximum string size in "vanilla" nodeJS (v0.10.28) is in the ballpark of 1GB.

If your are in a hurry, you can test the maximum supported string size with a self doubling string. The system tested has 8GB of RAM, mostly unused.

x = 'x';
while (1){ 
     x = ''+x+x; // string context
     console.log(x.length);
}

2
4
8
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65536
131072
262144
524288
1048576
2097152
4194304
8388608
16777216
33554432
67108864
134217728
268435456
536870912
FATAL ERROR: JS Allocation failed - process out of memory
Aborted (core dumped)

In another test I got to 1,000,000,000 with a one char at a time for loop.

Now a critic might say, "wait, what about JSON. the question is about JSON!" and I would shout THERE ARE NO JSON OBJECTS IN JAVASCRIPT the JS types are Object, Array, String, Number, etc.... and as JSON is a String representation this question boils down to what is the longest allowed string. But just to double check, let's add a JSON.stringify call to address the JSON conversion.

Code

x = 'x';
while (1){ 
     x = ''+x+x; // string context
     console.log(JSON.stringify({a:x}).length);
}

Expectations: the size of the JSON string will start greater than 2, because the first object is going to stringify to '{"a":"xx"}' for 10 chars. It won't start to double until the x string in property a gets bigger. It will probably fail around 256M since it probably makes a second copy in stringification. Recall a stringification is independent of the original object.

Result:

Pretty much as expected....

Now these limits are probably related to the C/C++ code that implements JS in the nodeJS project, which at this time I believe is the same V8 code used in Chrome browsers.

There is evidence from blog posts of people recompiling nodeJS to get around memory limits in older versions. There are also a number of nodejs command line switches. I have not tested the effect of any of this.

Is there a limit on the size of a string in JSON with Node.js?

Tags:

Javascript

Json

Node.Js

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