Node.js:如何使用循环引用序列化大对象
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【中文标题】Node.js:如何使用循环引用序列化大对象【英文标题】:Node.js: How to serialize a large object with circular references 【发布时间】:2015-12-24 04:24:21 【问题描述】:我使用 Node.js 并希望将大型 javascript 对象序列化到 HDD。该对象基本上是一个“哈希图”,只包含数据,不包含函数。该对象包含具有循环引用的元素。
这是一个在线应用程序,因此该进程不应阻塞主循环。在我的用例中,非阻塞比速度重要得多(数据是内存中的实时数据,仅在启动时加载,保存用于每 X 分钟和关机/故障时的定时备份)
最好的方法是什么?非常欢迎指向执行我想要的库的指针。
【问题讨论】:
Node.js 文件系统 API 默认是异步的,所以你需要问的问题是“如何使用循环引用序列化对象”? s/文件系统/IO。 NodeJS 中任何类型的 I/O 的默认设置都是异步的;您必须在标准库中的每种情况下明确要求同步执行。 你说的都是对的。问题是关于整个过程(javascript 对象 -> HDD -> 对象),但也许正确的问题确实是 joews 所建议的。 标题已更新。 到底有多大?如果你不能在一个部分中序列化它,你将在可变性方面遇到巨大的问题。 【参考方案1】:我有一个很好的解决方案,我一直在使用。它的缺点是它的运行时间为 O(n^2),这让我很难过。
代码如下:
// I defined these functions as part of a utility library called "U".
var U =
isObj: function(obj, cls)
try return obj.constructor === cls; catch(e) return false; ;
,
straighten: function(item)
/*
Un-circularizes data. Works if `item` is a simple Object, an Array, or any inline value (string, int, null, etc).
*/
var arr = [];
U.straighten0(item, arr);
return arr.map(function(item) return item.calc; );
,
straighten0: function(item, items)
/*
The "meat" of the un-circularization process. Returns the index of `item`
within the array `items`. If `item` didn't initially exist within
`items`, it will by the end of this function, therefore this function
always produces a usable index.
Also, `item` is guaranteed to have no more circular references (it will
be in a new format) once its index is obtained.
*/
/*
STEP 1) If `item` is already in `items`, simply return it.
Note that an object's existence can only be confirmed by comparison to
itself, not an un-circularized version of itself. For this reason an
`orig` value is kept ahold of to make such comparisons possible. This
entails that every entry in `items` has both an `orig` value (the
original object, for comparison) and a `calc` value (the calculated, un
circularized value).
*/
for (var i = 0, len = items.length; i < len; i++) // This is O(n^2) :(
if (items[i].orig === item) return i;
var ind = items.length;
// STEP 2) Depending on the type of `item`, un-circularize it differently
if (U.isObj(item, Object))
/*
STEP 2.1) `item` is an `Object`. Create an un-circularized version of
that `Object` - keep all its keys, but replace each value with an index
that points to that values.
*/
var obj = ;
items.push( orig: item, calc: obj ); // Note both `orig` AND `calc`.
for (var k in item)
obj[k] = U.straighten0(item[k], items);
else if (U.isObj(item, Array))
/*
STEP 2.2) `item` is an `Array`. Create an un-circularized version of
that `Array` - replace each of its values with an index that indexes
the original value.
*/
var arr = [];
items.push( orig: item, calc: arr ); // Note both `orig` AND `calc`.
for (var i = 0; i < item.length; i++)
arr.push(U.straighten0(item[i], items));
else
/*
STEP 2.3) `item` is a simple inline value. We don't need to make any
modifications to it, as inline values have no references (let alone
circular references).
*/
items.push( orig: item, calc: item );
return ind;
,
unstraighten: function(items)
/*
Re-circularizes un-circularized data! Used for undoing the effects of
`U.straighten`. This process will use a particular marker (`unbuilt`) to
show values that haven't yet been calculated. This is better than using
`null`, because that would break in the case that the literal value is
`null`.
*/
var unbuilt = UNBUILT: true ;
var initialArr = [];
// Fill `initialArr` with `unbuilt` references
for (var i = 0; i < items.length; i++) initialArr.push(unbuilt);
return U.unstraighten0(items, 0, initialArr, unbuilt);
,
unstraighten0: function(items, ind, built, unbuilt)
/*
The "meat" of the re-circularization process. Returns an Object, Array,
or inline value. The return value may contain circular references.
*/
if (built[ind] !== unbuilt) return built[ind];
var item = items[ind];
var value = null;
/*
Similar to `straighten`, check the type. Handle Object, Array, and inline
values separately.
*/
if (U.isObj(item, Object))
// value is an ordinary object
var obj = built[ind] = ;
for (var k in item)
obj[k] = U.unstraighten0(items, item[k], built, unbuilt);
return obj;
else if (U.isObj(item, Array))
// value is an array
var arr = built[ind] = [];
for (var i = 0; i < item.length; i++)
arr.push(U.unstraighten0(items, item[i], built, unbuilt));
return arr;
built[ind] = item;
return item;
,
thingToString: function(thing)
/*
Elegant convenience function to convert any structure (circular or not)
to a string! Now that this function is available, you can ignore
`straighten` and `unstraighten`, and the headaches they may cause.
*/
var st = U.straighten(thing);
return JSON.stringify(st);
,
stringToThing: function(string)
/*
Elegant convenience function to reverse the effect of `U.thingToString`.
*/
return U.unstraighten(JSON.parse(string));
;
var circular =
val: 'haha',
val2: [ 'hey', 'ho', 'hee' ],
doesNullWork: null
;
circular.circle1 = circular;
circular.confusing =
circular: circular,
value: circular.val2
;
console.log('Does JSON.stringify work??');
try
var str = JSON.stringify(circular);
console.log('JSON.stringify works!!');
catch(err)
console.log('JSON.stringify doesn\'t work!');
console.log('');
console.log('Does U.thingToString work??');
try
var str = U.thingToString(circular);
console.log('U.thingToString works!!');
console.log('Its result looks like this:')
console.log(str);
console.log('And here\'s it converted back into an object:');
var obj = U.stringToThing(str);
for (var k in obj)
console.log('`obj` has key "' + k + '"');
console.log('Did `null` work?');
if (obj.doesNullWork === null)
console.log('yes!');
else
console.log('nope :(');
catch(err)
console.error(err);
console.log('U.thingToString doesn\'t work!');
整个想法是通过将每个对象直接放入数组中来序列化一些循环结构。
例如如果你有这样的对象:
val: 'hello',
anotherVal: 'hi',
circular: << a reference to itself >>
那么U.straighten
就会产生这个结构:
[
0:
val: 1,
anotherVal: 2,
circular: 0 // Note that it's become possible to refer to "self" by index! :D
,
1: 'hello',
2: 'hi'
]
只是一些额外的说明:
我已经在各种情况下使用这些功能很长时间了! 非常不太可能存在隐藏的错误。
O(n^2) 运行时问题可以通过将每个对象映射到唯一哈希值(可以实现)的能力来解决。 O(n^2) 性质的原因是必须使用线性搜索来查找已经循环化的项目。因为这种线性搜索发生在一个已经线性的过程中,所以运行时间变成了 O(n^2)
这些方法实际上提供了少量的压缩!相同的内联值不会在不同的索引处出现两次。内联值的所有相同实例都将映射到相同的索引。例如:
hi: 'hihihihihihihihihihihi-very-long-pls-compress',
ha: 'hihihihihihihihihihihi-very-long-pls-compress'
变成(U.straighten
之后):
[
0:
hi: 1,
ha: 1
,
1: 'hihihihihihihihihihihi-very-long-pls-compress'
]
最后,如果不清楚,使用此代码非常容易!您只需要查看U.thingToString
和U.stringToThing
。这些函数的用法与JSON.stringify
和JSON.parse
的用法完全一样。
var circularObj = // Some big circular object you have
var serialized = U.thingToString(circularObj);
var unserialized = U.stringToThing(serialized);
【讨论】:
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