feat: Optimise langchain calls in batching mode (#15243)

packages/@n8n/nodes-langchain/nodes/chains/TextClassifier/processItem.ts

@@ -0,0 +1,57 @@
+import type { BaseLanguageModel } from '@langchain/core/language_models/base';
+import { HumanMessage } from '@langchain/core/messages';
+import { ChatPromptTemplate, SystemMessagePromptTemplate } from '@langchain/core/prompts';
+import type { OutputFixingParser, StructuredOutputParser } from 'langchain/output_parsers';
+import { NodeOperationError, type IExecuteFunctions, type INodeExecutionData } from 'n8n-workflow';
+
+import { getTracingConfig } from '@utils/tracing';
+
+import { SYSTEM_PROMPT_TEMPLATE } from './constants';
+
+export async function processItem(
+	ctx: IExecuteFunctions,
+	itemIndex: number,
+	item: INodeExecutionData,
+	llm: BaseLanguageModel,
+	parser: StructuredOutputParser<any> | OutputFixingParser<any>,
+	categories: Array<{ category: string; description: string }>,
+	multiClassPrompt: string,
+	fallbackPrompt: string | undefined,
+): Promise<Record<string, unknown>> {
+	const input = ctx.getNodeParameter('inputText', itemIndex) as string;
+
+	if (input === undefined || input === null) {
+		throw new NodeOperationError(
+			ctx.getNode(),
+			`Text to classify for item ${itemIndex} is not defined`,
+		);
+	}
+
+	item.pairedItem = { item: itemIndex };
+
+	const inputPrompt = new HumanMessage(input);
+
+	const systemPromptTemplateOpt = ctx.getNodeParameter(
+		'options.systemPromptTemplate',
+		itemIndex,
+		SYSTEM_PROMPT_TEMPLATE,
+	) as string;
+	const systemPromptTemplate = SystemMessagePromptTemplate.fromTemplate(
+		`${systemPromptTemplateOpt ?? SYSTEM_PROMPT_TEMPLATE}
+	{format_instructions}
+	${multiClassPrompt}
+	${fallbackPrompt}`,
+	);
+
+	const messages = [
+		await systemPromptTemplate.format({
+			categories: categories.map((cat) => cat.category).join(', '),
+			format_instructions: parser.getFormatInstructions(),
+		}),
+		inputPrompt,
+	];
+	const prompt = ChatPromptTemplate.fromMessages(messages);
+	const chain = prompt.pipe(llm).pipe(parser).withConfig(getTracingConfig(ctx));
+
+	return await chain.invoke(messages);
+}